Long Trains and Couplers

[PRR8259]

Model train wheels generally (excepting the use of "scale" wheelsets) would actually have a larger width of contact area on the rails than prototype tapered wheelsets do.

This alone will dramatically increase the amount of friction to overcome for a train to move.  So it is not possible to scale down prototypical numbers to HO.

Also, although model locomotives running downhill theoretically are braked by the gears, this does not hold true in practice, as the weight of a train behind them can actually push the gears slightly faster.  Didn't any of you who are operating long trains ever notice that they actually go a bit faster on downgrades than on the level?  It may not be a lot but it definitely occurs.

Do you really mean to tell me that a second engine can't sometimes push the front engine to run faster than it normally does just by itself, despite the gearing?  It does happen.

John

[ATLANTIC CENTRAL]

Tin Can II

 

 

ATLANTIC CENTRAL

 

 

 

 

 

 

 

The way to prevent long trains from "dwarfing the layout" is to rethink your approach to layout design.

In a moderately large, or even medium sized space, rather than trying to model multiple "towns", which are then heavily selectively compressed, and too close to each other, you can just model one town, model it on a grander scale, with less compression, then model a few miles of open country on either side, some interchanges and junctions, all leading to lots of staging.

Move all the other "towns" off stage, or nearly so.

Now your long trains will look properly proportioned to the scenes. And you will capture more of the "immensity" of the prototype.

 

 

My new layout will model one small western Maryland city, and few of its "suburbs" and nearby countryside. The entire area behind the freight yard will be urban scenery, the rest will be suburban and rural scenery.

A single 25' long freight yard is easier and less expensive to build than two 12' long ones at each end of a layout, and is way more realistic looking.

Use this rule, model each feature only once, and model it bigger and better.

Just a thought.

To my eye, trains look like the local wayfreight until you get up around 35-40 cars. Then they start to look like mainline trains.

The average train on my layout will require 3-4 diesels, or two moderate size steam locos. Some will require more or bigger power.

Sheldon

 

 

 

 

Sheldon:  I like your approach to layout design.  I intend to do something similar with my main line junction; featuring a full sized yard and interchange, and then running to staging in both directions.

 

Here is my trackplan, and there is thread on here with more details.

 

Sheldon

[Tin Can II]

ATLANTIC CENTRAL

 

 

 

 

 

 

 

The way to prevent long trains from "dwarfing the layout" is to rethink your approach to layout design.

In a moderately large, or even medium sized space, rather than trying to model multiple "towns", which are then heavily selectively compressed, and too close to each other, you can just model one town, model it on a grander scale, with less compression, then model a few miles of open country on either side, some interchanges and junctions, all leading to lots of staging.

Move all the other "towns" off stage, or nearly so.

Now your long trains will look properly proportioned to the scenes. And you will capture more of the "immensity" of the prototype.

 

 

My new layout will model one small western Maryland city, and few of its "suburbs" and nearby countryside. The entire area behind the freight yard will be urban scenery, the rest will be suburban and rural scenery.

A single 25' long freight yard is easier and less expensive to build than two 12' long ones at each end of a layout, and is way more realistic looking.

Use this rule, model each feature only once, and model it bigger and better.

Just a thought.

To my eye, trains look like the local wayfreight until you get up around 35-40 cars. Then they start to look like mainline trains.

The average train on my layout will require 3-4 diesels, or two moderate size steam locos. Some will require more or bigger power.

Sheldon

 

Sheldon:  I like your approach to layout design.  I intend to do something similar with my main line junction; featuring a full sized yard and interchange, and then running to staging in both directions.

[SeeYou190]

Overmod

All this really proves...

That is not what any of this proves at all.

-Kevin

[BATMAN]

Lastspikemike

That was my point. Early railway builders were also canal builders. The rolling resistance of a canal boat increased by a lot when the canal builders included a grade....

That got me thinking of all those paddle wheelers heading up the Fraser to Yale.

Sometimes they had to throw them a rope.

[Overmod]

All this really proves only the phenomenal efficiency of a hard wheel on a hard rail with low bearing resistance.  I don't expect to see anyone running a THOUSAND CARS up a continuous 2% any time soon... did anyone bother to reality-check that little detail?  Of course the resistance goes through the roof with even slight gravity... look at the trailing weight!  (Happens with the prototype, too, with a lot fewer cars in the train... )

84oz seems not unreasonable for pulling about a tenth of a REAL mile of continuous model railroad cars.

[ATLANTIC CENTRAL]

Railroad Facts and Figures
Copyright AA Krug

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Tractive Effort vs Horsepower

Many people confuse Horsepower (Hp) and Tractive Effort (TE). With this essay I hope to clear that confusion. I use some terms and formulas in this essay that may not be familiar. I suggest that you read the Definitions at the end of this document.

Assume a train that weighs 15,000 tons is stopped on a 1% grade. For every ton of train weight on a 1% grade a force of 20 pounds is acting to roll the train down the hill. A 15,000 ton train produces a force of 300,000 lbs. (15,000 tons x 20 lbs per ton = 300,000 lbs).

To prevent this train from rolling back down the hill we must apply an equal force in the opposite direction to the coupler of the first car. Imagine yourself holding onto the coupler of the first car and trying to hold the train from rolling back down the hill. It should be obvious that try as you might you will not be able to hold the train. No matter how hard you grasp the coupler you cannot hold the train because your shoes will simply slide across the ties and ballast. The adhesion of your shoes to the ties and ballast is not equal to 300,000 lbs. Thus not even if you were Superman could you develope 300,000 lbs of pull (traction) to hold the train.

The level of force required to break the adhesion of your shoes to the ties and thus slide your shoes is dependent upon two things.
1. The coefficient of friction between your shoes and the ties.
2. The weight on your shoes.
Increase either one of those parameters and your adhesion to the ties increases enabling you to pull harder before sliding.

For the purpose of this article we will say the factor of adhesion of a steel wheel on a steel rail is about 30%. (Actually it is more like 20%-25% but we will get to that later). A factor of adhesion of 30% means that a wheel will stick to the rail so that a pull equal to 30% of the weight on that wheel is required to break that adhesion and slide the wheel. Locomotives have a lot of weight on their wheels. A typical 4 axle GP40 might weigh 280,000 lbs. Four of them weigh 1,120,000 lbs. Thirty percent of 1.12 million pounds equals 336,000 lbs. In other words, it requires a force greater than 336,000 lbs to slide four GP40s. Therefore these locomotives, with their brakes set, will hold the train on the hill since the weight of that train on the grade is only producing 300,000 lbs of pull.

Tractive Effort is the amount of Pull.
You can have pull with no HP. Notice that in the example above we are applying a force, a pull, a Tractive Effort, of 300,000 lbs to that coupler of the first car but we don't need any diesel engines. All we need is at least 1 million pounds of weight on the wheels and a factor of adhesion of 30%. We have Tractive Effort (TE) without any Horsepower (HP).

If I don't need any HP why did I use 4 locomotives. Why not just use one very heavy loco? Because the maximum weight on any single wheel that steel rails can withstand is 35,000 lbs. If we put more weight than that on any wheel it will crush the rail head and create excessive wear or outright failure of the rail. The two wheels on opposite ends of one axle can each carry 35,000 lbs of weight. So the maximum weight on an axle assembly is 70,000 lbs. Since we need a total locomotive weight of at least 1 million pounds that means we need a minimum of 14.3 axles. Three 4 axle locos would only be 12 axles so we must use four which gives us 16 axles.

From this discussion we can see that total locomotive weight determines the maximum amount of tractive effort a locomotive can produce. The number of wheels or the number of traction motors has nothing to do with it. We only add additional wheels to spread the required weight out along the rails to avoid damaging the track. I said that 3 GP40s will not work in this case because they only have 12 axles and each can be weighted no more than 70,000 lbs for total locoweight of 840,000 lbs. At 30% adhesion that only produces 252,000 lbs of TE and we need at least 300,000 lbs. If we take those same 3 GP40s and add two more axles to each unit but DO NOT increase the weight of each unit, then we still only have 840,000 lbs of total weight and 30% of that is still only 252,000 lbs. Adding axles and wheels to locomotives does not increase tractive effort.

HP is the Tractive Effort (pull) times the Speed.
Burn that statement into your brain. It is crucial to understanding this essay.
While our coal train is just sitting on the grade, held there by the locomotives, there is 300,000 lbs of "pull" on the first car's drawbar. Because the train is not moving there is no HP required. But try to move it at 1 mph up that hill and HP is required. The required HP is the TE needed for the grade (300,000 lbs) times the speed (1 mph or 1.47 ft per second) divided by the definition of a HP (550 lb-ft per second).

(300,000 lbs) x (1.47 ft per sec) / (550 lb-ft per sec) = 801 HP

The HP required is 801 HP! Yes just 800 hp will move this coal train up the hill. Amazing isn't it? But will one 800 HP 4 or 6 axle unit do it? No! Because that one 800 hp unit must have at least 1 million pounds on its drivers to prevent it from sliding back down the hill. You must have the weight to get the adhesion required. That means each wheel of an 800 hp 6 axle unit would have to have 84,000 lbs on it. (That is 168,000 lbs. per axle). Oh my the crushed and broken rails that would leave behind! Not to mention the overloading of bridges and the track structure itself. As I said above, the minimum number of axles we need to spread out the required weight is 14.3 axles. It doesn't matter whether we have four 200 Hp 4 axle units, or whether we have one 800 Hp 4 axle unit and three 4 axle engineless slugs . It is all the same to the coal train.

One mph is kind of slow. It would take us 24 hours just to get up Parkman hill. We want to go up the hill about 15 mph. 15 mph is a good compromise between taking forever and extreme high power costs. To go up this hill at 15 mph (22 ft per second) requires:

(300,000 lbs) x (22 ft per sec) / (550 lb-ft per sec) = 12,000 HP

Gee now that sounds familiar doesn't it. That is exactly the Hp of four 3,000 Hp units! So we can use four 3000 Hp GP40s. SD70MACs have 6 axles and weigh 420,000 lbs. Again 70,000 lbs per axle. So three of them give us 18 axles and 1.26 million pounds on the wheels, more than enough to produce our required 300,000 lbs of traction. SD70MACs are rated at 4000 Hp so three of them are 12,000 Hp. Four GP40s or three SD70s, either way you satisfy both the adhesion needed and the HP needed.

Horsepower Alone.
Will two 6000 HP units work? No. While you have the required 12,000 hp you can only have 70,000 lbs weight per axle, which times the 12 axles is only 840,000 lbs total. With 30% adhesion and only 840,000 lbs of weight the two 6000 HP units have only 252,000 lbs of adhesion. Not enough to hold or move the coal train. The weight of the train on the grade will slide these two locomotives backwards down the hill. Remember you need at least 300,000 lbs of adhesion (traction).

High HP locomotives on 6 axles create other problems such as traction motor (TM) overheating. Can we pour 1,000 hp into each TM continuously at 10-15 mph without frying them? Low gearing helps. But low gearing lowers the top speed because the TMs will fly apart at high rpms. Here is where AC locos have an advantage over their DC counterparts. AC TM rotors are much more solid than DC TM armatures so they can be geared lower and still have a high top end.

Varying Adhesion.
All of the above figures are based on a 30% adhesion factor. IE, the wheels grip the rail with a force equal to 30% of the loco weight. Locomotives of the GP40/SD40 era and their Dash 2 offspring are generally considered to have an adhesion factor of about 25% not 30%. Sand will increase this factor to about 30%. Thus to achieve the 30% adhesion needed for the examples used in this essay so far, these locos may need to be on sanded rail.

Modern locos such as SD70MACs and C44s claim adhesion factors of 36 to 43%! They do this by using sophisticated anti-wheelslip circuits. These circuits allow the wheels to spin slightly faster than the rail speed warrants. It is called creep. Strangely enough, a creeping wheel has a higher factor of adhesion than a stationary or rolling wheel. Thus in theory two 6,000 HP SD90s weighing 420,000 lbs each and achieving an adhesion factor of 36% will produce a TE of 302,400 lbs and should pull the train up the hill at 15 mph.

However, in my experience you cannot count on that 36% adhesion factor in all types of weather and rail conditions. On wet or frosty rail these units slip and you stall. And when you stall you had better set the train airbrakes in a hurry or the train will slide these units back down the hill. On the other hand I have had C44s, SD90s and SD70MACs absolutely astound me with what they are pulling. At times they attain greater than 40% adhesion on dry, sanded, rail. It is that "at times" that concerns me. You cannot count on them to do that reliably time after time.

An Actual Experience with High HP Locos and Adhesion.
One night I was running a freight up hill at 7 mph with a Dash 9-44CW on the point. I had previously calculated that we should have gone up the hill at 11 mph, so why were we only doing 7 mph? The rail was slightly frosty. I punched up the loco monitor screen on the computer. It showed that this supposedly 4400 Hp unit was only putting out 2930 HP!!! It had derated to prevent slipping in spite of the sanders being on. So the adhesion factor of this loco at that time was not the touted 36-43% but instead only 22%. The railroad had paid for a 4400 HP locomotive with 36% adhesion but was only getting a 2930 HP locomotive with 22% adhesion. The common SD40-2 would have done as good or better in this situation than the hi-tech wonder. This was not a one time occurrance. I have seen similar performances on many occasions.

Horsepower is Speed.
Up to now we have assumed that a locomotive has enough power to slip its wheels. That is true only at low speeds. Note that Hp is TE times speed. If the speed remains the same and the TE (pull) increases then the Hp requirement increases. If the TE remains the same and the speed increases then the Hp requirement increases. If you have a fixed maximum Hp, such as a loco has, then as speed increases the TE must come down. The product of the two must remain a constant and is directly related to the HP rating of the loco. On the 12,000 Hp coal train above why can't we go faster than 15 mph on the 1% grade? Because 15 mph times the required 300,000 lb of drawbar pull divided by 550 lb-ft per second (the definition of a HP) equals the total locomotive Hp. If the train went faster the product of the speed times the pull would be higher and thus the required Hp would be higher. But we are limited to 12,000 hp on this consist so it trudges along at 15 mph. Similarly if we throttle down a notch or two, reducing the HP, then the speed is going to drop because there is now less than 12,000 HP available. The drawbar pull account of the grade remains the same at 300,000 lbs and the lower HP means a lower pull x speed figure so the speed must drop until the product is proportional to the new lower HP. This why I like to say Hp is speed.

Steeper Grades.
We are proceeding at 15 mph up the 1% grade with our 15,000 ton train powered by four 4 axle GP40s. What happens when we encounter a steeper grade with this train? The drawbar pull needed to hold or move a train on a grade is 20 lbs per ton per grade percent. That is where the 300,000 lb figure came from for the 1% grade of the example above. If this train were to roll onto a 1.5% grade what happens? The drawbar pull needed now is 450,000 lbs.

(15,000 tons) x (20 lbs per ton) x (1.5 grade) = 450,000 lbs

We still have only 12,000 hp available. Since HP equals pull times speed, if the pull goes up then the speed must come down. The train speed will drop to 10 mph, the point where the product of the new 450,000 pounds pull and the speed divided by 550 (HP definition) equals the available HP. But we are in real serious trouble here folks. Our four 4 axle locos can only deliver 336,000 lbs of pull because of their 1.12 million lbs of weight on drivers and the 30% adhesion factor. So our locos are going to slip and stall on the hill. Although we have enough Hp to pull this train up the 1.5% grade at 10 mph we do not have enough traction to do so. And, again, you had better set the train air brakes as you stall or the train will drag you back down the hill.

Slugs, and SDs.
So how do we proceed? Well we can't increase the weight on drivers by adding weight to our existing four 4 axle locos because they are already weighted to the max for the rail. The only other way to increase traction, weight on drivers, is to increase the number of drivers. Add more units. We will need to add two more units (8 more axles) to get the weight up to at least 1.5 million lbs so the 30% adhesion factor gives us 450,000 lbs of adhesion. We do not need the added HP of those two units however, they could be just engineless slugs. Without additional Hp the train will go up the hill at 10 mph. At this point I would like to point out that if we simply add two 4 axle slugs, which get the power for their traction motors through electrical cables from the original 4 locos, that we can do the same thing by switching from GP models to SD models. From 4 axle units to 6 axle units. An SD40 is simply a GP40 with two more axles (with traction motors) and 50% more weight. In other words we've added "half a slug" to each of the 4 units. In this manner we once again have the required weight on drivers by using just four SD40s. These are the same weight and number of drivers as 4 four axle units plus two 4 axle slugs. If we want to go up the hill at 15 mph instead of 10 mph however, we must add the Hp. Slugs or converting to SDs will not do. If the additional units added are 3000 hp like the rest then we will again go up the hill at 15 mph. Hp is speed.

Helpers.
In either case we will not go up the hill very far, probably not at all! Why not? The figures say we will. The one word is Kapow! You are going to break in two. We are now trying to put 450,000 lbs of pull into a coupler rated at 390,000 lbs, it is going to break. So what can we do? Well we could double the hill. Take half the train up to the top and leave it there then come back with the engines and get the second half. When you get both halves to the top, recouple them, make an air test, and proceed. By taking half the train up the hill at a time the required coupler pull is only half that of the entire train or only 225,000 lbs. Well under the 390,000 lb strength of our couplers. This method also requires no more slugs, SDs, or other units. The original 4 GP40s have enough traction to haul half the train up at a time. Unfortunately doubling the hill requires a lot of time. The line is blocked while it is being done and this train and others are delayed for the duration.

Alternatively we could put the added two units on the rear of the train to PUSH. We would need another engineer (a helper engineer) or a distributed power set-up (radio controlled slaves). The physics are the same, the coal train and grade could care less where you apply the power just so you have the right amount to move it. But the couplers do care where you put all that power. If you try to put it all thru one coupler, the first one, it is going to say "Screw you, I ain't gonna take this abuse" and it will break to prove its point. (You never knew couplers were so animated did you.)

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Some things left out.

Now for you purists, I know I have left out a few things. (If you know enough about this to know that I left things out then you sure don't need to be reading this document).

·         As the grade gets steeper less and less of the loco weight is felt as pressing directly down on the rails so effective weight on drivers decreases slightly. (Do the geometry yourself if you want).

·         I neglected the weight of the locomotives. They don't go up hill for free.

·         I neglected the efficiency of the locomotive's mechanical and electrical transmission.

·         I neglected rolling resistance of the train. At low speeds such as these, on straight rail, rolling resistance for a loaded coal train is only 10-15% relative to the grade resistance. However that will increase the total pull and HP required.

·         I neglected acceleration. The figures given are for steady state running. To accelerate requires more pull than steady speed.

The Weight of the Locomotive.

First we'll look at the loco weight. Four 280,000 lB GP-40-2s weigh 1,120,000 lbs. That 1 million pounds of locos does not go up the hill for free. It takes just as much HP to move each of those pounds up the hill as each of the train's pounds. So you should add their weight to the train when calculating traction, speeds, & Hp required. In fact this is one reason for 4 axle high HP locos. The more the locos weigh the more of their Hp is required to just to move the loco upgrade.

Heavy Haul vs High Speed.
You may have noticed that most railroads tend to use 6 axle power on heavy trains such as coal and grain while they use 4 axle power on their high speed lighter weight intermodal trains.
1. If a particular railroad has a good mix of high speed and heavy haul trains its locomotive roster will be a mixed bag of 4 axle and 6 axle power.
2. If a railroad, such as BN, has a preponderance of coal and grain trains and/or operates its trains in mountainous territory where grades are steep and speeds are low its roster will be dominated by heavy 6 axle power.
3. If a railroad, such as ATSF, has a majority of lighter weight high speed intermodal trains and a lot of relatively flat territory its roster will reflect that with lots of high Hp 4 axle power and/or lighter weight 6 axle power.

Lets look at the difference between 6 axle SD40s and 4 axle GP40s. Suppose we want to run a 5200 ton priority manifest train up a 1% grade at 30 mph. This requires 8320 HP. TE is not a consideration because even three 280,000 lb GP40s will have 252,000 lbs of adhesion. Our train only requires 104,000 lbs of adhesion on this grade. Three GP40s weigh a total of 840,000 lbs or 420 tons. These locos require 672 Hp just to move themselves up the grade at 30 mph. So our 5200 tons of train requires 8320 Hp and the 420 tons of locos require 672 Hp to go up this grade at 30 mph, total 8,992 Hp. The three GP40s produce 9,000 Hp. What happens if we use three SD40s instead of GP40s. Same Hp at 3000 each but the SDs weigh much more. Ours are ballasted for lots of TE needed on coal & grain trains. Our SD40s weigh 420,000 lbs each. Three of them weigh 630 tons! To move these SDs up the grade at 30 mph requires 1008 HP. This means we only have 7992 HP left for the train. That means we can only haul 4995 tons at 30 mph instead of the original 5200 that the GP40s hauled. While this may not seem like much difference it is over 4% and a 4% efficiency improvement is a big deal when you are burning 1.6 billion gallons of fuel per year. Trains that run at high speeds don't need heavy locos with lots of Tractive Effort. What they need are light weight high horsepower locos.

Here is a table showing the theoretical Tractive Effort a 3000 Hp loco produces at various speeds.

Speed	Tractive Effort
60 mph	18,707 lbs
40 mph	28,060 lbs
30 mph	37,415 lbs
25 mph	44,898 lbs
15.0 mph	75,000 lbs
13.4 mph	84,000 lbs
8.9 mph	126,000 lbs

Using a 30% adhesion factor, a locomotive that weighs 280,000 lbs has 84,000 lbs of adhesion. From the above chart we can see that such a loco can operate as slow as 13.4 mph without slipping. Suppose our railroad has a large proportion of service sensitive intermodal trains and we want to operate those trains no slower than 30 mph on our worst grades. At 30 mph a 3,000 Hp loco is only capable of producing 37,415 lbs of Tractive Effort. Therefore as long as we put enough Hp on our trains to maintain 30 mph on our worst grades we do not need to make them weigh 280,000 lbs. In fact they only need to weigh about 125,000 lbs because 30% of 125,000 is 37500 lbs of adhesion and we only need 37,415 lbs. We could save a lot of fuel by using these light weight locos instead of the 280,000 lbs heavy weights. If we have four such locos on a train we are saving 310 tons of wasted weight and that translates into saved fuel.

A loco that cannot use full throttle below 30 mph without slipping would be rather restricted in its service. So railroads tend to compromise. If instead of 280,000 lb locos or 125,000 lb locos we use a loco weighing 250,000 lbs we still get some of the fuel savings and the loco becomes much more versatile since it can now operate as low as 15 mph in full throttle without slipping.

It is in high speed freight service where locos with high Hp to weight ratios shine. This why the ATSF had 3800 Hp 4 axle GP60s and 4,000 Hp 4 axle B40-8s. Since BN had a preponderance of heavy coal & grain trains and even its freights had to contend with steep grades it had 3,800 Hp heavy SD60s and 4,000 Hp heavy SD70MACs instead of the GP60s and B-40-8s that the ATSF had.

Lets look at this Hp to loco weight ratio from another angle. The following chart shows the maximum Tractive Effort of various loco models and the speed at which that maximum TE is achieved. All units are 3,000 Hp and we assume a 30 % adhesion factor.

Model	Weight	Max TE	Speed	Total tons on 1% grade	Trailing tons
Light GP40	250,000	75,000	15.0	3750	3625
Heavy GP40	280,000	84,000	13.5	4200	4060
Light SD40	380,000	114,000	9.8	5700	5540
Heavy SD40	420,000	126,000	8.9	6300	6090

A first glance at the table looks as if the heavy SD40 is the best loco. It can pull the most trailing tonnage up the 1% grade. But we have to ask ourselves "What is the job?". If the job is to haul as much tonnage up the grade as is possible, then indeed the heavy SD40 is the loco we want. But if the job is to haul as much trailing tonnage up the grade at 15 mph then the SD40 is not the best choice. The following chart shows how much tonnage each of these locos can haul up the 1% grade at 15 mph. Since all the locos are 3,000 Hp they all produce the same 75,000 lbs of TE at 15 mph. But the weight of the loco uses up some of that TE. What is left over can pull the freight that is paying the bills.

Model	Weight	TE	Speed	Total tons on 1% grade	Trailing tons
Light GP40	250,000	75,000	15	3750	3625
Heavy GP40	280,000	75,000	15	3750	3610
Light SD40	380,000	75,000	15	3750	3590
Heavy SD40	420,000	75,000	15	3750	3540

The light weight GP40s can haul 85 more tons of paying freight per unit up the grade at 15 mph than the heavy SD40 can.

Those Superpower units.
If you haven't been paying attention you might think that the new 6000 HP single unit locos are destined for heavy haul service. True they are all heavy 6 axle units. But that is because the weight is needed to put that 6,000 HP to the rail without slipping. A 6,000 Hp unit that weighs 420,000 lbs and can attain a 43% adhesion factor has an adhesion of 180,600 lbs. The 6,000 Hp diesel engine can deliver that 180,600 lbs of Tractive Effort at a speed of 13 mph. Below that speed you cannot use full throttle on these locos because they will slip. That was for an astounding adhesion factor of 43%. What if they cannot maintain that extreme level of adhesion? What if they "only" get 36%? 36% of 420,000 lbs is 151,200 lbs of TE. The 6000 hp diesel can deliver that TE at 15 mph so the loco cannot operate below 15 mph in full throttle without slipping. At an adhesion factor of 30% the lowest full throttle speed is 18 mph. If the rail is wet or frosty can these modern marvels maintain even a 30% adhesion factor? My experience with 4400 Hp units is a definite no. The C44s often have trouble maintaining 22% adhesion with bad rail conditions. If a 6,000 Hp unit gets down to 22% adhesion it can only operate at full throttle above 24 mph! Thus if you want these behemoths to reliably move your trains over the hills in all kinds of weather you had better dispatch them with trains light enough that they can maintain 24 mph or greater on your steepest hills. That means they are only useful for trains such as intermodals which get a high Hp to tonnage ratio. When it is frosty they won't work on heavy freights or coal or grain trains which routinely pull up the hills at 10-12mph.

The railroad I work for uses 12,000 Hp on their coal trains through here and we go up the hills at about 12-13 mph. Note that you can replace the 12,000 Hp of 3 SD70MACs, or the 12,000 Hp of 4 SD40-2s, with the 12,000 Hp of just two SD90s. You have the same Hp so you should go up the hills at the same 12-13 mph. But it will be awfully iffy. That is because the minimum speed these 6,000 Hp units can operate at full throttle is 13 mph even with an adhesion factor of 43%. If anything causes the train speed to fall below 13 mph even momentarily, you will never regain the lost speed. The train might be temporarily slowed for various reasons. Perhaps the SD90s temporarily lost that 43% adhesion factor and slipped or reduced Hp to prevent slipping. Perhaps a wind came up and increased train restance. At 12 mph the 6,000 Hp locos cannot operate in full throttle even if they regain that 43% factor of adhesion. They will slip. Operating at reduced throttle the locos are not producing the 12,000 Hp this train needs to travel up the hill at 13 mph. So the train will never accelerate back up to 13 mph where it could again operate at full throttle. Four SD40s or 3 SD70MACs would have no difficulty re-accellerating the train back up to 13 mph. That is because they are not operating at the limit of their adhesion as the SD90s are. The 4 SD40s have 12,000 Hp just like the two SD90s but the SD40s have a total weight of 1,680,000 lbs and even at a 30% factor of adhesion can operate in full throttle down to 9mph! The 3 SD70MACs weigh 1,260,000 lbs and with only a 30% factor adhesion they can operate at full throttle down to 11.9 mph. If they achieve a 36% factor of adhesion they can operate at full throttle down to 9.9 mph. So either the SD40s or the SD70s have enough reserve adhesion they can operate at full throttle after being temporarily slowed. That allows them to accellerate the train back up to the 13 mph.

Thus on an equal total Hp basis these high Hp units are not equal to their lower Hp cousins when used in heavy haul service. And heaven help you (more like helpers help you) if the factor of adhesion on these brutes ever falls below 36% because you won't have enough adhesion to pull that 15,000 ton train up that 1% grade, period. You had better hope that it does not rain, frost, or snow.

Keep the high Hp units in high speed freight service where they do the most good. You are trading 8 axles of weight on two 3000 HP GP40s or 12 axles of weight on two SD40s for the 6 axles of the new units and you have 25-50% less Hp-wasting weight with the two high Hp units. Remember that TE decreases as speed increases, so as long as they keep the Hp per ton ratio of the trains high enough to maintain high speeds then the TE will be low enough that these high Hp single units won't slip. But try to use them in low speed drag service and they will slip as noted in the coal train discussion above. The slower the train goes up a hill the closer these high powered 6,000 Hp wonders perform like the good old 3,000 Hp SD40.

The Efficiency of the Locomotive.

Next we'll look at the efficiency of the locomotives transmission. Their transmission consists of the generator, traction motors, and gearing. My experience is that the loco's transmission efficiency normally runs in the 80% range. This means that if the physics of the train, grade, and speed dictate X HP then you really need X / .80 HP. If the physics say 12,000 HP then you really need 12,000 /.80 which is 15,000 HP. Another full unit!
Put another way....The 15,000 ton coal train going up a 1% grade at 10 mph requires 9564 HP. That is 8442 HP for the speed up that grade and 1122 Hp for the rolling resistance at that speed. (We'll get to rolling resistance in a minute). But that assumes 100% efficiency. At 80% efficiency this train would need 9564 Hp / .80 which is 11,995 Hp. SURPRISE! That is three 4,000Hp SD70MACs or four 3,000Hp SD40-2s to get a 15,000 ton coal train up a 1% grade at 10 mph. Sound familiar?

The Rolling Resistance of the Train.

Now we'll look at rolling resistance. Assume the same train as above, ie., 15,000 tons plus 840 tons of locos (4 SD40-2s) rolling at 10 mph on a 1.0% grade. Using the well known Davis formula we get the following values:

Resistance	Pull	HP
Grade	316,800 LBs	8447 HP
Rolling	41,880 LBs	1116 HP
Total	358,680 LBs	9563 HP

The calculated Hp required is 9563 Hp. Since our locos are only about 80% efficient this means we need a Hp rating of 12,000 to actually deliver the required 9563 Hp.

Put the train on a 2000 ft long 3 degree curve and you get:

Resistance	Pull	HP
Grade	316,800 LBs	8447 HP
Rolling	41,880 LBs	1116 HP
Curve	15,840 LBs	422 HP
Total	374,520 LBs	9985 HP

Using a loco efficiency of 80% the required 9985 Hp becomes 12,481 Hp. The 12,000 Hp of four SD40s is not going to be able to pull this train up the hill and around the curve at 10 mph. The speed will drop until the rolling resistance and grade Hp drops enough that the actual Hp required equals 80% of 12,000 Hp (9600 Hp). That speed is 9.6 mph.

At 9.6 mph we get the following values:

Resistance	Pull	HP
Grade	316,800 LBs	8109 HP
Rolling	41,557 LBs	1063 HP
Curve	15,840 LBs	405 HP
Total	374,197 LBs	9577 HP

 

Accelleration.

Lets look at accellerating trains. The force of acceleration is mass times acceleration. Force = mass x acceleration. A coal train is a _very_ big mass! So even small accellerations need a lot of force. That force adds to the drawbar pull account of the grade alone and it can break the train in two. If you keep the acceleration low by notching out one notch at a time and allowing speed to increase slowly you can minimize the force of acceleration. If you are reckless and try to accelerate quickly you may end up in two pieces.

Lets say we want to accelerate a 15,000 ton train at a rate of 1 mph per minute. In other words we want to be going 1 mph faster at the end of one minute than we are going now. To accellerate at that rate requires a steady force of 23,450 lbs. Note that it doesn't matter whether we are going up hill or on the level. We need to supply an additional 23,450 LBs of drawbar pull to accellerate at 1 mph per minute. Horsepower is pull times speed. Since the force to accellerate this train at 1mph/min is a constant, the HP required to accelerate the train varies according to speed. At 10 mph the HP needed is 625 Hp, at 40 mph the Hp needed is 2500 Hp.

An accelleration of 1 mph/minute is slow. It would take a train 60 minutes to go from 0 to 60 mph. But if we want to accelerate at 10 mph per minute it requires 10 times the force and 10 times the Hp at each speed. At an accelleration rate of 10 mph per minute the drawbar force needed is 234,520 lbs. At 10 mph that requires 6,250 Hp. At 20 mph it requires 12,500 Hp. At 40 mph it requires a whopping 25,000 Hp. Note that if we have only 12,000 Hp then we run out of Hp before we reach 20 mph. We can no longer accelerate at 10 mph per minute and will fall back to lower and lower acceleration rate as speed increases.

Keep in mind that these values of drawbar pull and Hp are ONLY for acceleration. You still need to supply the normal pull and Hp for any grade and rolling resistance. Lets look at that. A 15,000 ton train on a 1% grade going 8 mph requires 357,104 lbs of pull and 7617 Hp. If we have 4 SD40-2s we have 12,000 Hp times 80% efficiency = 9600 Hp available. 7620 Hp is what you get in throttle 7. We have one more throttle notch and 1980 Hp (9600-7620) remaining that we can use for acceleration. At the stated 8 mph that equates to 97,345 lbs of additional drawbar pull available. This additional force will accelerate the train at a rate of almost 4 mph per minute. Yeehaw! Put 'em in number 8 throttle and we'll be doing 12 mph at the end of the next minute.

Well not quite. A few problems crop up with that assumption. One is that as the train speed increases so does the horsepower required for both the grade and the rolling resistance. So as the speed begins to increase we have less "left over HP" for acceleration. The rate of acceleration will drop, we cannot maintain that 4 mph / minute rate we started with. In fact when we reach 10 mph all of the loco's HP is being used to pull the train and none is left over for acceleration. The speed will level out at 10 mph and stay there. Ain't physics neat?

The second problem is that you just broke the train in two so you are actually stopped. Why? Because the train traveling at 8 mph required 357,104 lbs of drawbar pull to maintain that speed on this grade. When you opened the throttle from notch 7 to notch 8 to accelerate you just put the additional 92,602 lbs of available loco tractive effort into that same drawbar. 357,104 lbs + 92,602 lbs equals a total of 449,706 lbs. Since drawbars are only good for about 390,000 lbs you just pulled one in two. Moral: When you are moving slowly you'd better handle that throttle gently if you want the train to remain in one piece. Acceleration can break trains in two.

Now for the purists, those 4 SD40-2s are not going to develope that 454,449 lbs of TE. That would mean an adhesion factor of 27% and SD40-2s can rarely if ever achieve that. They would most likely slip. But they can develope the 390,00 lb rating of the drawbars either continuously or by slipping and jerking. So either way the train is going to be in two pieces.

Note that the above train theoretically can go up this hill at 10 mph based upon Hp, efficiency, grade, and drawbar strength. Whether it actually can or not is in doubt. If we rolled onto this hill at a speed higher than 10 mph then all would be OK. As the train rolled onto the grade in number 8 throttle it would simply slow down to 10 mph and proceed up the hill. The drawbar force would be that 357,104 lb figure. Well within the rating of the drawbars. But if the train had stopped on this grade or had started from a stop on a lesser grade and was not yet up to 10 mph then we may be in trouble. Under these circumstance we may find ourselves in the situation above where we are only going 8 mph when the entire train is on the hill. We cannot go from notch 7 to notch 8 because the drawbar force will exceed their rating. Thus we cannot get to 10 mph. The only recourse is to slug it out all the way up the hill in the lower throttle position and a lower speed. It is very annoying to know you have the HP to go faster but you can't use it. If you are a real good engineer and really know what you are doing you can get around this obsticle in some cases. How? By applying some independent brakes to the locomotives drivers. Those brakes will absorb some of the extra Hp you get when you go from #7 to #8. Therefore that amount of the extra Hp and its attendent TE never reaches the train's drawbars. In that manner you can keep the total drawbar force lower than the drawbar rating. As the speed increases you feather off more and more of the independent brake until finally you are at the 10 mph physical limit and the brake is fully released. But make one mistake during that process, fail to coordinate the independent brake just right with the increasing Hp as the locos rev up and increase their load....or feather it off too quickly.......and Kapow! You are in two pieces. You have let enough extra TE reach the drawbars that their rating was exceded. Going 10 mph in #8 vs 8 mph in #7 saves you 15 minutes on the hour. But if you break it in two attempting to reach 10 mph then you are delayed 2 hours while you chain up a car and set it out and double the hill. If you are not sure of your expertise maybe it is better to just go up the hill at 8 mph in number 7 instead of trying for 10 mph in number 8.

The main point of all this is to hopefully dispell the myth that high HP means lots of pull. It does not. Higher HP means higher pull at higher speeds but the total maximum pull is strictly related to weight on drivers. No HP required. None! Therefore a switch engine which only operates at low speeds does not need, nor can it use, high HP. It needs to be heavy. (but not too heavy that it breaks or turns over light industrial or yard rails). Life is a compromise.

Whew!

<="" a="">

<="" a="">Definitions.

<="" a="">

as used in this document

<="" a="">

·         <="" a=""><="" a="">Axle - Two wheels and an axle with a traction motor geared to it. All "axles" are powered, there are no idler axles.

·         <="" a="">GP40 - A 4 axle locomotive of 3,000 Hp that weighs 280,000 lbs.

·         <="" a="">SD40 - A 6 axle locomotive of 3,000 Hp that weighs 420,000 lbs.

·         <="" a="">Slug - A 4 axle unit that has traction motors but no diesel engine. Its traction motors get their electrical power from adjacent units. A concrete weight ballasts the slug to 280,000 lbs.

·         <="" a="">C44- A 6 axle locomotive of 4,400 Hp that weighs 420,000 lbs. Actual model designation is Dash 9-44CW.

·         <="" a="">SD70MAC - A 6 axle locomotive of 4,000 Hp that weighs 420,000 lbs and has AC traction motors.

·         <="" a="">SD90 - A 6 axle locomotive of 6,000 Hp that weighs 420,000 lbs and has AC traction motors.

·         <="" a="">Grade Pull or Grade Resistance - The force required on a grade to prevent a train from roling back down the hill. It is expressed as 20 lbs per ton per percent of grade.

·         <="" a="">Adhesion - The ability of the steel wheels of a locomotive to "stick" to the steel rails to prevent spinning or sliding of the wheels. The amount of force required to slide the wheels of a locomotive.

·         <="" a="">Adhesion Factor - The ratio of the adhesion to the weight of a locomotive. A good ballpark figure for steel wheels on steel rails is 30%. IE, it requires a force equal to 30% of the loco's weight to slide its wheels.

·         <="" a="">Tractive Effort - The pull developed by a locomotive. The maximum tractive effort value is directly proportional to the weight on drivers and the adhesion.

·         <="" a="">Horsepower - Any combinaton of pull and speed that equals 550 lb-ft per second. Examples: Pulling with a force of 1 pound for 550 feet and accomplishing that in one second. Pulling with a force of 550 lbs for 1 foot and accomplishing that in 1 second. Pulling with a force of 225 lbs for 2 feet and accomplishing it in 1 second. Etc.

<="" a="">

<="" a="" style="color: rgb(0, 0, 0); font-family: "Times New Roman"; font-size: medium; font-style: normal; font-variant-ligatures: normal; font-variant-caps: normal; font-weight: 400; letter-spacing: normal; orphans: 2; text-align: start; text-indent: 0px; text-transform: none; white-space: normal; widows: 2; word-spacing: 0px; -webkit-text-stroke-width: 0px; text-decoration-thickness: initial; text-decoration-style: initial; text-decoration-color: initial;">

[DrW]

Lastspikemike

The calculations you performed demonstrate that although the proportion of coupler loading contributed by grade  is significant the total added force isn't very much.

I am confused. In agreement with my approach via work/energy, the force calculations by Jerry show that a 2 % gradient increases the force needed to pull his hypothetical train from 4 oz to 84 oz. You call this "not very much"? Or do you imply that, in absolute values, 80 oz (the difference between 4 oz and 84 oz) is "not very much"? Have you ever tried if an HO coupler can support a weight of 5 lb?

[hjQi]

I read through the posts and got confused. Some says at a grade the resistance just increases slightly, so the loco can pull more than 2%. On a first look, it makes sense as we only increase 2% of the resistance. But my experience is that most of loco would have hard time to pull if the grade is more than 4%. What's wrong?

So I did some simple math or physics.

Assume the rolling resistance is Crr=0.001 (According to Wiki for metal wheels). Each car is W=4oz. We are pulling N=1000 cars.

On a flat track, the required pulling force is

F=Crr*N*W=0.001*1000*4=4oz

On a track with 2% grade, the required force is

F=Crr*N*W+W*N*G=4+1000*4*0.02=84oz

The second term is the weight contribution to the pulling direction due to the inclined angle.

jlwii2000 tested the recent scaletrain and intermountain ET44AC, the pulling force of these two locos are about 4.9oz and 4.4oz. So let's say a diesel has a pulling force of 4oz. So on a flat track, one loco can pull 1000 cars. But on a 2% grade, you would need 21 locos, or a loco can only pull 50 cars. On a 4% grade, you would need 41 locos, or a loco can only pull 25 4oz cars. The number of locos needed would increase significantly or the number of cars that can be pulled by a single loco would decrease significantly if you consider other factors, such as most cars are heavier than 4oz, 0.001 is actually the lowest number for Crr, you have curves on your track...

On a flat track, the loco only needs to overcome the rolling resistance. But on a graded track, the loco needs to overcome the weight, like DrW said, the potential energy.....

Jerry

 

[ATLANTIC CENTRAL]

BATMAN

 

So, when a long HO scale train is descending a hill with a lot of cars does the engine(s) start bobsledding at some point? If the wheels keep turning what is going on with the motor? is this dynamic braking? Would there be anything disruptive or damaging going on? Would a decoder get confused? What else is going on?  Once again my tiny brain wants to know. 

Generally speaking, all other speculation by others aside, if a loco, or group of locos, has the necessary tractive effort to pull the train up the hill, it likely has enough adheasion to hold the train still on the hill.

Are those equations always equal? No. And the protoype pays close attention to that. 

But I have never had a string of cars, not even 100 of them in a 2% helix, push a loco down hill faster than the wheels are turning.

Sheldon

[kasskaboose]

For what most home layouts, I'd think a 15-20 consist would operate fine even on a 2% grade behind one loco all with metal couplers. 

My cars are 7-8" long because I operate the early 1980s. Correct that I should be fine when behind only loco even going up/down a 2% incline? 

I agree with others that an extremely long consist (50+) doesn't look very realistic on many home layouts.

[csxns]

BATMAN

long HO scale train is descending a hill with a lot of cars does the engine(s) start bobsledding

That happens on my layout and it is DC only.

[BATMAN]

https://www.youtube.com/watch?v=-hPuNFjRTZA&ab_channel=JamesRisner 

I did notice a couple of spots with mid-train helpers.

Not too long ago, I had my three Rapido F units haul a 72 car train away from an area I was working on to keep them away from my troublesome elbows. I parked it with the locomotives just over the crest of a 42' long hill that averages a tad less than 2%. It sat there all day and then whamo! A coupler gave way and the look of horror on my face must have been something as I watch some 70 cars heading for what I surely thought would be the floor. The rate they picked up speed was noteworthy and the actual speed they got to was considerably higher than what I would ever run a train at. In the end, the whole works held fast to the rails with not one derailment.

So, when a long HO scale train is descending a hill with a lot of cars does the engine(s) start bobsledding at some point? If the wheels keep turning what is going on with the motor? is this dynamic braking? Would there be anything disruptive or damaging going on? Would a decoder get confused? What else is going on?  Once again my tiny brain wants to know.

https://www.youtube.com/watch?v=PAQUahxY7XY&ab_channel=JamesRisner

[maxman]

A number of years ago, in an issue I cannot find at the moment, MR did a comparison of many of the available HO couplers.  As part of this testing they did a test to see how much force was required to break/deform the coupler being tested.

The Kadee coupler could not be tested to failure bcause its strength exceeded the capacity of the testing device.

[SeeYou190]

BATMAN

For argument's sake, let's say we are using good Kadee couplers and have a layout with modest 2% ups and downs and 40" radius curves. There are other variables I am probably not including, so how many cars as an average do you think an HO train could have before suffering coupler breaks under the weight?

Look at the nonsense you started Brent.

In conclusion, your question is meaningless in the real world. 2% grades would require a train so heavy to break couplers that it would be nearly impossible to get enough locomotives to pull that hard.

Anyway, there are plenty of YouTube videos of ridiculously long trains and couplers do not break.

-Kevin

[SeeYou190]

Well, that makes no sense at all. Lots of words just to lead to this incoherent conclusion.

-Kevin

[DrW]

Let's try physics. The force you need to pull an object against rolling resistance is

f = m g C(rr)

where m is the mass of the object, g the gravitational constant, and C(rr) the rolling resistance coefficient. The energy (work) needed to pull this object over the distance d is

E = m g C(rr) d

If you have a gradient, you have to put in additional work in form of potential energy, E(p), to get the object up the height h:

E(p) = m g h

For a 2 % gradient, h is obviously 2 % of the distance or 0.02 d, giving

E(p) = 0.02 m g d

m, g, and d are the same in both equations. Thus, the question is how large is C(rr)? According to Wikipedia, the values for (prototype) railroad wheels on steel rails fall between 0.0010 and 0.0024, with a value of 0.0020 given for a railroad passenger car. This means that the energy needed to overcome the 2 % gradient is about 10 times as large as the energy needed to overcome the rolling resistance. This is why railroads need helpers on an incline. Admittedly, I do not know what the C(rr) value for model trains is, but I cannot imagine that it is an order of magnitude larger than for the prototype. 

[ATLANTIC CENTRAL]

John-NYBW

I have industrial belt lines that run on both sides of the track between my classification yard and main city. It provides ample opportunities for switching. I think my favorite job though is the wayfreight which serves the other two towns and then continues on into staging to theoretically serve more. The middle of the three towns is the smallest and has only two industries to serve. Just before reaching the last of the three towns, there is a dummy interchange which serves as a universal industry. Within the last town are 3 industries as well as an interchange with a short line that is currently under construction. There is a small yard there to handle the interchange traffic with the short line which hopefully will soon be operational. 

Although I designed my yard with an A/D track that can handle 50+ car trains, from a practical stand point, 20 cars is the typical manifest freight. I use a car card/waybill system and occasionally that results in trains a bit longer but I can't ever remember going over 25 cars during normal operations. A 20-25 car train looks plenty long in HO. It's more than my eye can take in at once which makes it seem longer than it actually is.

 

John, again your layout sounds very nice and well thought out for operations. 

I was simply pointing out that there are ways to run longer trains and not have them overwhelm the layout, even a moderate sized layout. 

You are correct, trade offs and compromises, which do you want to make. My choices worked well for me because I like the double track action.

Double track, combined with the "one citiy" theory, does a lot to make the layout feel bigger than it is and make longer trains more at home.

Those choices to do not suit or interest everyone, agreed and understood.

The other "choice" gets back to the idea of having most destinations off the layout, or one the layout. Some really like the idea of seeing the origin/destination of rolling stock both on the layout. For me that is not and issue, as an operator or a viewer.

My passenger operations do include one large thru terminal and five small "suburban" and rural stations to allow good commuter train and "rural local" operation in addition to long distance trains arriving/departing the main terminal.

My goals include the ability to have both advanced opps sessions, and very good high action display running.

Sheldon

[selector]

Lastspikemike

... Grades cannot increase rolling resistance. Curves can but not grades.

...

Mike, let us perform a thought experiment.  You and I are both standing beside a bench where a 3/4" plywood plank is elevated at one end, the lower resting on the bench surface, such that the elevation constitutes a 2% grade.

I couple two small HO ore jennies together and ask you to grasp the lead coupler and draw the two cars up the incline.  I ask you to note the apparent effort.  No need to measure it exactly.

Then, I couple sixteen jennies of the same type, all with the same rolling resistance in the wheelsets due to friction and fit, and I ask you to repeat the process.  Would you expect to have to exert yourself considerably more to draw all those coupled cars up the grade?  I believe you would.  That is rolling resistance, and it's due to gravity. If you were to release the lead car, what would happen?  The cars would accelerate back down the incline.  This is what the engine must contend with.  It isn't reasonable to say that gravity doesn't amount to rolling resistance. Otherwise, why have more horsepower?  And, if you put a sufficient horsepower on the head end, say six SD-70M at Run 8, and trail them with 20K coupled tons, all but one coupler at 100% designed capacity, the one at 99%, would you expect any breakage?  I believe you should, and we both know which it would be in all probability.  Now trail the same tonnage, same horsepower, on a 2% grade.  Would you expect the one coupler to part even more easily?  I believe you should.  See -  HP vs TE

[doctorwayne]

I don't see that broken Kadee couplers could be all that much of an issue.  The only ones that I've experienced were on locos or rolling stock that decided to take a face-first closer look at the layout room's concrete floor.

Lastspikemike

Just a note to remind all that a 2% grade doesn't add much weight to the force on the couplers. The maths is fairly easy.

It may not have much influence on the couplers, but it definitely affects the locomotive's pulling abilities.
A loco that can handle a heavy train on level track can be "on-it's-knees" on a 2% grade, and there's no need for math, other than perhaps the number "2" or even "3" (representing a 2nd or 3rd locomotive).

BATMAN

PRR8259, thank you for that excellent contribution. I took high school physics but that was it, you gave us food for thought presenting more variables than I would have thought of or considered.

I second your "thank you" Brent.

Lastspikemike

The effect of the grade will be to add a small proportion of force from the exertion of gravity on the mass of the train, minus the locomotive mass of course. Grades cannot increase rolling resistance. Curves can but not grades.

 

Apparently, you've not run a heavy train on a grade, nor one that's also on a grade with a number of curves.  I suggest you give it a try, as it may help with your mathematical calculations.

 

Wayne

 

[BATMAN]

PRR8259, thank you for that excellent contribution. I took high school physics but that was it, you gave us food for thought presenting more variables than I would have thought of or considered. Now if Lastspikemike will bring forth his simple enough math I may be even more informed, or not.

[PRR8259]

Lastspikemike

A 2% grade doesn't add much force to the couplers. Particularly model train couplers. But the maths is easy enough if someone wants to do the work. 

The OP postulated a 2% grade which someone pointed out as if it makes a difference to the issue of coupler failure. It doesn't and cannot. The total maximum force the locomotive can exert declines by exactly the same proportion.

The grade isn't relevant  to the underlying conceptual question actually posed. If a model locomotive can successfully move a train on the level without breaking any coupling links then it can haul that same train up a grade without breaking those same links. At least that would be my guess, not having done the maths. 

The underlying conceptual question, as yet not clearly identified, is can a model locomotive or consist of model locomotives exert enough force to break a model train apart?  If so, what is the likely failure point? 

 

I completely disagree with your premise and conclusions.

I am a practicing, licensed professional civil engineer (which means I don't just do the work but am responsbile for the plans;  I sign and seal them, and can then be sued if something is wrong).

While it may appear on the surface that a 2% grade does not increase the mass or weight all that much, you are completely neglecting the affects of metal wheel on metal rail friction, which adds an "effective" increase to the weight of any train (and the force on couplers), which in real life, with 10,000+ ton trains on a 2% grade like Cajon Pass, becomes very significant.  This is why some above commented that grade DOES affect pulling power.  You are increasing the EFFECTIVE weight as the grade increases.  You also are dramatically increasing the effective weight that must be braked on the downgrade due to acceleration of the train pushing on the engines.

Additionally, any horizontal curvature at all also increases friction drag between the wheel flanges and the rail.  In real life there are mathematical equations that can estimate the increased friction based upon horizontal curvature.

Real railroads do the math and then assign maximum tonnages allowed over a division, and it varied quite a bit based upon the grade and horizontal curvature.

Drawbars are more likely to break with slack run in and run out forces if train movement is not carefully controlled.

Even in the model world, poor rolling wheelsets dramatically increase the pulling forces on the couplers in long trains because they dramatically increase friction forces.  Most Kadee couplers are not going to fail barring a manufacturing defect, but plastic ones may.

The free-rolling qualities of the rolling stock being pulled can make a tremendous difference in what one's engines are able to pull.  When non-free-rolling freight cars are in a train, they dramatically increase the drawbar pull required to move the train.  It is not even the weight issue so much as the friction issue.  Just try pulling an 80 car train by the coupler only using two fingers.  The cars may not weigh much but there's an awful lot of friction to overcome.

In most cases, the model engines will fail to move the model train well before the metal couplers break.  This is because the pulling power of the engines is more the limiting factor than the couplers.

I'm not even going to try scaling down equations to HO.  There are too many variables in the model world that can affect train performance.

[John-NYBW]

ATLANTIC CENTRAL

 

 

John-NYBW

Everything in this hobby is a tradeoff. Having multiple towns increases operational possibilities while fewer towns allows for more longer open country runs. I've been building my layout for the last 20 years and it's a little late to be rethinking the layout design. I have one large city and two smaller towns on the mainline with staging yards at either end. I think that strikes a good balance. My towns have a decent amount of distance between them. I could eliminate the middle town to create a longer open country run but I prefer having one more place where my passenger trains stop and my local freight has some switching to do. 

Just outside my main city is my large classification yard. It has a 12 track ladder with the longest track being 25 feet. I designed it with the idea of maybe running 50 car freights but when I saw what that looked like, I decided it was just too much. 20 car freights look plenty long in my space. On rare occasions I might even stretch it out to 25. My freights generally run a single steamer or an AB set of F units. I have one ABB set for my fast freight but one of the units requires repair so right now it is also an AB set. I have lots of passenger trains, probably too many, and they are pulled by a variety of equipment from AB E units to RS-1s on the commuter trains. I have several streamlined Hudsons as well. 

As for compressing the towns, I don't feel I've done that at all. All three have backdrops which make them seem much larger than they actually are especially my largest city which is about 12 feet long but has a backdrop running the full length and gives it a lot of depth. To enter the staging yard, my trains pass under a mirror which doubles the apparent size of the city as well as the structures that butt up against it. 

 

 

 

John, I would not disagree with any of that and your layout sounds very nice.

I think the biggest difference is I took almost all the industries off the mainline, and put them on industrial branch lines or belt lines, mostly in the one city modeled.

There are a few more remote industries out on the main, but there are four  separate industial areas all associated with the main yard and city, two do not require even going on or across the main for operations, one is on the main, the other only requires crossing the main. 

So by the standards most layouts are built to, that is four "towns" worth of industries, and four local trains that have little effect on what is happening on the mainline.

I don't expect to run many trains larger than 40 or 50 cars, but for my eye that is large enough to require three and four unit diesel lashups, and doublehead lots of steam, especially in a region that is in the piedmont, leading to the mountains.

My staging is all thru staging, the mainline is continious double track, which also helps improve the appearence of long trains - no worrying about siding length vs distance between sidings.

Sheldon 

 

I have industrial belt lines that run on both sides of the track between my classification yard and main city. It provides ample opportunities for switching. I think my favorite job though is the wayfreight which serves the other two towns and then continues on into staging to theoretically serve more. The middle of the three towns is the smallest and has only two industries to serve. Just before reaching the last of the three towns, there is a dummy interchange which serves as a universal industry. Within the last town are 3 industries as well as an interchange with a short line that is currently under construction. There is a small yard there to handle the interchange traffic with the short line which hopefully will soon be operational. 

Although I designed my yard with an A/D track that can handle 50+ car trains, from a practical stand point, 20 cars is the typical manifest freight. I use a car card/waybill system and occasionally that results in trains a bit longer but I can't ever remember going over 25 cars during normal operations. A 20-25 car train looks plenty long in HO. It's more than my eye can take in at once which makes it seem longer than it actually is.

[mvlandsw]

According to Sam at Kadee their couplers are made of pure zinc.

I had a Kadee coupler fail one time. I was running about 50 cars down a 2% grade when a truck screw near the rear of the train worked its way out. It got caught in a switch frog and brought the train to a sudden stop, at least that part of the train behind the knuckle that snapped.

Real trains can break more than one coupler at the same time. If the brakes apply suddenly on the rear of the train a group of cars can break off while the rest keeps moving. When enough braking force builds up on what's left another section can separate. This can repeat a number of times. I heard of one instance where four knuckles were broken.

Mismatched coupler heights are the cause of most model train separations which can lead to frightening runaways on grades. I saw this happen once on a live steam operation. The size of that equipment can lead to serious injury or even death in the resulting pileup. Some of that equipment has safety chains between the cars like older prototype passenger car used to have.

Mark

[ATLANTIC CENTRAL]

John-NYBW

Everything in this hobby is a tradeoff. Having multiple towns increases operational possibilities while fewer towns allows for more longer open country runs. I've been building my layout for the last 20 years and it's a little late to be rethinking the layout design. I have one large city and two smaller towns on the mainline with staging yards at either end. I think that strikes a good balance. My towns have a decent amount of distance between them. I could eliminate the middle town to create a longer open country run but I prefer having one more place where my passenger trains stop and my local freight has some switching to do. 

Just outside my main city is my large classification yard. It has a 12 track ladder with the longest track being 25 feet. I designed it with the idea of maybe running 50 car freights but when I saw what that looked like, I decided it was just too much. 20 car freights look plenty long in my space. On rare occasions I might even stretch it out to 25. My freights generally run a single steamer or an AB set of F units. I have one ABB set for my fast freight but one of the units requires repair so right now it is also an AB set. I have lots of passenger trains, probably too many, and they are pulled by a variety of equipment from AB E units to RS-1s on the commuter trains. I have several streamlined Hudsons as well. 

As for compressing the towns, I don't feel I've done that at all. All three have backdrops which make them seem much larger than they actually are especially my largest city which is about 12 feet long but has a backdrop running the full length and gives it a lot of depth. To enter the staging yard, my trains pass under a mirror which doubles the apparent size of the city as well as the structures that butt up against it. 

John, I would not disagree with any of that and your layout sounds very nice.

I think the biggest difference is I took almost all the industries off the mainline, and put them on industrial branch lines or belt lines, mostly in the one city modeled.

There are a few more remote industries out on the main, but there are four  separate industial areas all associated with the main yard and city, two do not require even going on or across the main for operations, one is on the main, the other only requires crossing the main. 

So by the standards most layouts are built to, that is four "towns" worth of industries, and four local trains that have little effect on what is happening on the mainline.

I don't expect to run many trains larger than 40 or 50 cars, but for my eye that is large enough to require three and four unit diesel lashups, and doublehead lots of steam, especially in a region that is in the piedmont, leading to the mountains.

My staging is all thru staging, the mainline is continious double track, which also helps improve the appearence of long trains - no worrying about siding length vs distance between sidings.

Sheldon 

[John-NYBW]

ATLANTIC CENTRAL

 

 

John-NYBW

I have a fairly large basement layout. My steepest uphill grade is 1.5% while the downhill grade is 1.75%. I generally limit my freight cars to 20 cars but early on I tried to see if I could run a 50 car freight. The couplers held up with no problem. The operational problem I had is there is a curve near the bottom of the grade and I superelevated it slightly. I found that all that weight tended to pull the cars over the inside rail.

The other problem is a train that long dwarfs the layout. The locos on the front end would pass through a town while the caboose had yet to reach the town behind it. 

I've since reduced the amount of superelevation so the rails are near level on that curve so I might try the experiment again just to see if I get better results, but 20 car freight will continue to be the max. My freelanced layout is loosely based on the NYO&W and 20-30 car freights were not unusual on that line. That's probably why it went belly up.  

 

 

 

 

John, 

The way to prevent long trains from "dwarfing the layout" is to rethink your approach to layout design.

In a moderately large, or even medium sized space, rather than trying to model multiple "towns", which are then heavily selectively compressed, and too close to each other, you can just model one town, model it on a grander scale, with less compression, then model a few miles of open country on either side, some interchanges and junctions, all leading to lots of staging.

Move all the other "towns" off stage, or nearly so.

Now your long trains will look properly proportioned to the scenes. And you will capture more of the "immensity" of the prototype.

 

My new layout will model one small western Maryland city, and few of its "suburbs" and nearby countryside. The entire area behind the freight yard will be urban scenery, the rest will be suburban and rural scenery.

A single 25' long freight yard is easier and less expensive to build than two 12' long ones at each end of a layout, and is way more realistic looking.

Use this rule, model each feature only once, and model it bigger and better.

Just a thought.

To my eye, trains look like the local wayfreight until you get up around 35-40 cars. Then they start to look like mainline trains.

The average train on my layout will require 3-4 diesels, or two moderate size steam locos. Some will require more or bigger power.

Sheldon

 

Everything in this hobby is a tradeoff. Having multiple towns increases operational possibilities while fewer towns allows for more longer open country runs. I've been building my layout for the last 20 years and it's a little late to be rethinking the layout design. I have one large city and two smaller towns on the mainline with staging yards at either end. I think that strikes a good balance. My towns have a decent amount of distance between them. I could eliminate the middle town to create a longer open country run but I prefer having one more place where my passenger trains stop and my local freight has some switching to do. 

Just outside my main city is my large classification yard. It has a 12 track ladder with the longest track being 25 feet. I designed it with the idea of maybe running 50 car freights but when I saw what that looked like, I decided it was just too much. 20 car freights look plenty long in my space. On rare occasions I might even stretch it out to 25. My freights generally run a single steamer or an AB set of F units. I have one ABB set for my fast freight but one of the units requires repair so right now it is also an AB set. I have lots of passenger trains, probably too many, and they are pulled by a variety of equipment from AB E units to RS-1s on the commuter trains. I have several streamlined Hudsons as well. 

As for compressing the towns, I don't feel I've done that at all. All three have backdrops which make them seem much larger than they actually are especially my largest city which is about 12 feet long but has a backdrop running the full length and gives it a lot of depth. To enter the staging yard, my trains pass under a mirror which doubles the apparent size of the city as well as the structures that butt up against it. 

[ATLANTIC CENTRAL]

crossthedog

 

 

ATLANTIC CENTRAL

My new layout will model one small western Maryland city, and few of its "suburbs" and nearby countryside.

 

 

@Sheldon, my folks both sprang out of Maryland, my mom from Pikesville and my dad from Reisterstown. The history of my dad's family is all around Taneytown, Frizzelberg, Mayberry, Uniontown. When I'm in Carroll County, I can't swing a dead cat without hitting a dead relative's gravestone. My mom fondly remembers running to the end of her street as a girl to see the WM steam locomotives chuffing through the ravine on their way out of Bawlmer to points north and west.

 

-Matt

 

I'm originally from Anne Arundel County, now in Harford County for the last 26 years. But being a life long Marylander who has traveled all over the state to work construction, I know all those places you mentioned pretty well.

I grew up in Severna Park, where I was lucky enough to be a member of the Severna Park Model Railroad Club, once again recently featured in Model Railroader.

And while the new layout freelanced, there is a direct effort to give it the feel of the western/northern counties. While no effort is made to model exact places, the place names are taken from the region with some "association" with the real places. 

Sheldon

[crossthedog]

ATLANTIC CENTRAL

My new layout will model one small western Maryland city, and few of its "suburbs" and nearby countryside.

@Sheldon, my folks both sprang out of Maryland, my mom from Pikesville and my dad from Reisterstown. The history of my dad's family is all around Taneytown, Frizzelberg, Mayberry, Uniontown. When I'm in Carroll County, I can't swing a dead cat without hitting a dead relative's gravestone. My mom fondly remembers running to the end of her street as a girl to see the WM steam locomotives chuffing through the ravine on their way out of Bawlmer to points north and west.

-Matt

[selector]

Lastspikemike

Just to remind everyone that the rails (and roadbed) hold up the weight. The coupler, and indeed the locomotive moving the train, only overcome the rolling resistance not the total weight of the train.

Indeed, the whole secret to the load carrying capacity of railroads is the low deflection of the rail and roadbed. 

Once the initial static friction is overcome, rolling resistance becomes quite low and not meaningfully related to train weight. 

Any force will accelerate any mass absent friction. Eventually you'll notice the mass moving.... 

 

You are neglecting the effect of gravity on a grade.  It is substantial.  See Al Krug's article on locomotive horsepower and what it takes to ascend a grade with trailing tonnage.  Just as a quick reference, for every 0.5% grade increase, it takes approximately 3.2 X the initial horsepower to ascend if maintaining the same track speed.  If you are willing to slow substantially, you can get away with generating less tractive effort/horsepower.

The couplers each have to withstand the draw forces on them.  Longer trains, or more horsepower stretching them, things start to break. Why else do we have 'distributed power' on trains going over mountain passes?  It's to save the crew from having to walk back with a 70 pound knuckle and replace it.