Guys, I really thank you for all the information and conversation that's gone on here. And very importantly, thank you to Balt for the information on the tonnage ratings on the Bow Line. That comes in handy to me since I'm planning to model the area one day. I've also been able to find an employee timetable from back in 05 which helps with siding lengths and such. Now that I'm armed with tonnage ratings too, I'm even better than I was.
Also, thanks to everyone for the information on sanding vs not sanding and everything else.
DeggestyHaving cut my teeth on ETT's about sixty years ago, were engine limitations then determined from experience?
The major RR's all had Dynamometer Cars back then. So they were able to actually test the performance of their engines on all of their lines. On the CPR, this led to the publication of a book outlining the tonnage each type of steam engine could haul on every line. This became known as an engine's "A" rating, and was then known by all employees who needed that information.
To this day, diesels are still said to have an "A" rating, although the explanation of what an "A" rating for a particular steam engine was, could only really be understood by a Boilermaker or a Power Engineer. As I understand it, the last time CP used its' Dynamometer Car (#62) was when they developed the procedures for DPU usage of SD40's in the late '60's. Since then all "A" ratings have been done as calculations based on the original test data.
Car #62 is now at the Canadian Railway Museum outside of Ottawa, still in servicable condition. It is there on a type of lend deal, where should the railway ever need it, they could still use it.
Bruce
So shovel the coal, let this rattler roll.
"A Train is a Place Going Somewhere" CP Rail Public Timetable
"O. S. Irricana"
. . . __ . ______
Some sidebar-like stuff:
Sanding....steam men at the start of the grade started sanding. With diesels, not a good idea; waiting 'til the speed dropped to where wheel spin was likely made more sense than dragging a train upgrade on sanded (higher rolling resistance) rail.
But when descending in high dynamic braking power, I sanded when I got near maximum amperage....Cause by the time the late 90's high HP, DC or AC, reloaded after recovering from a wheel slip, my whole plan for descent was garbage.
Power assignments: has anyone considered the "what's available" factor?
Say the planners, the devils back at Corridor Mgr HQ has nothing but thru' trains, minimally powered, goin' thru where you originate a train and you're rested.....no call. The local Yard-Monster informs you the power for your train left Nevada an hour ago; you're "on the shore of the San Francisco Bay" and the "flower in your hair" wilted and you feel like some of those "Raiders fans" at the Black Hole look.
Hp/t and coupler limits:
Grades and knuckle capability relate and rule. Tractive effort, speed, train or minimum speed rating don't. If at no point can the required power exceed a pulling power that exceeds in pounds of stress the knuckle metal can handle, the odds favor a "good trip."
It's 12 ounces per minute through each of the two nozzles on each of axles 1 and 4 or 3 and 6, depending on the direction in which the unit is headed. The problem is typically debris or moisture in the sand but can also be poor quality sand. A unit shouldn't run out of sand if it was sanded when it was last fueled. I expect -- though don't really know -- that sand flow is frequently less than it should be; however the reduced flow won't usually become apparent unless it contributes to a stall.
JayPotterCSXT's tonnage ratings are based on locomotives having an adequate flow of traction sand. The specification is 12 ounces per minute per sander.
Johnny,
I can not speak from experience but the relationship between rated TE and tonnage over a particular district was obvious far back in the steam era, even if figuring out TE by calculation may not have been obvious in the earliest days.
I have a copy of "Field Engineering - A Handbook of the Theory and Practice of Railway Surveying, Location and Construction", 22nd Ed., 1948. From the Prefaces to several editions, none of what I am about to discuss was modified after 1924. The first Edition came out in 1880.
Beginning at page 325 are several pages on "Theory of Maximum Economy in Grades and Curves" and it includes a discussion of rolling resistance, grade resistance, and curve resistance and compensation. The text mentions elaborate experiments about rolling resistance conducted by Professor E. C. Schmidt of the University of Illinois, and a footnote indicates that the results of these experiments were published in 1910.
Curve resistance was figured out fairly early since most mountain lines built later than about 1880 seem to have been compensated for curvature.
My best guess is that in the pre Civil War era tonnage rating of locomotives was pretty much a matter of experimentation, but by the 1920's it was by calculation with some test runs to prove the calculations.
Mac
I don't know the answer to Deggesty's question; but I suspect that the answer is yes, particularly at low speeds. It seems to me that the more performance depends on horsepower, the more accurately it can be calculated; but the more performance depends on adhesion, the less accurately it can be calculated.
A question: Having cut my teeth on ETT's about sixty years ago, were engine limitations then determined from experience? That was the impression I had back then.
Johnny
PNWRMNM Bama, The railroad operates first and foremost on tonnage. Conversion from tons to cars is imprecise since mixed freight cars have various weights depending on whether loaded or empty. Car count does become a limiting factor in terms of siding length. It is very handy to have your trains short enough to fit in the sidings. Mac
Bama,
The railroad operates first and foremost on tonnage. Conversion from tons to cars is imprecise since mixed freight cars have various weights depending on whether loaded or empty. Car count does become a limiting factor in terms of siding length. It is very handy to have your trains short enough to fit in the sidings.
PNWRMNM Sam, In general terms power assignment is done by train brief which typically gives tonnage and length limits plus power requirements. I think most roads assign power for manifest (carload) and intermodal on the basis of Horsepower per Ton (HPPT). A preferred carload train may get 3 HPPT, a stack train 2.5-3, and a hot trailer train 4-5 HPPT, all over the same territory. Drag freights and unit coal and grain trains are generally assigned based on published tonnage ratings, which as Balt said are just enough to get the train over the road. Just to make it interesting not all locos are created equal for drag service. DC units have a minimum continuous speed which is the lowest speed that they can be operated under full power for a long time. Think SD-40 and variants and a MCS of about 12 MPH. AC units do not have the MCS limit, they can be operated all day at 2 MPH without damage. This is big reason carriers are careful about mixing AC and DC units in the same consist since crawling up the hill at 5 MPH will damage DC traction motors. Of course if a train has enough HP, it can get up the hill at 12 MPH and all is well, but that negates the idea of absolute minimum HP on drag freight. BNSF operates 110 car export grain trains over Marias Pass MT 1% westward ruling grade. How much power do they need? The grade is 20#/ton resistance and everything else is say 4 #/ton. Train weighs 110 X 143 or 15,730 tons. AT 24 #/ton need Tractive Effort of AT LEAST 377,000 pounds. With SD-40 at 90,000# need 4.18, actually 5 units. With modern AC at say 142,000# TE I need 2.65 so I assign three and take the excess in speed greater than what the units were rated at. Couplers are all grade E which are rated for 360,000# IIRC. The power can put out more than that but train resistance should not be more than 377,000#. All involved hope there are no bad knuckles in the head end or the crew will find it and tie up the line for an hour or so, in good weather, to fix it. In practice the units will be rated at 5900 tons. Roundhouse foremen, dispatchers and crew will find they have capacity of 17,700 tons and a train of 15,730 tons so they are good to go. Of course since they do this day in and day out no need to really think about it until or unless the railroad is power short or a unit dies or some other disturbance occurs. Actual units are usually selected by Roundhouse Foreman based on expected tonnage from yardmaster, dispatcher, or centralized power desk. Practice varies by railroad, by locality, and by time in history. What the computer spits out is load, empties, tons, and feet based on what the clerk shows on the train which is based on what yardmaster and switch foremen told the clerk. Train will be scanned out of the yard which should correct consist errors. Mac PS at 8:32 PM IIRC BNSF is actually operating DPU mode, 2 on the point and one on rear, to limit draft forces in head part of the train. Mac
Sam,
In general terms power assignment is done by train brief which typically gives tonnage and length limits plus power requirements. I think most roads assign power for manifest (carload) and intermodal on the basis of Horsepower per Ton (HPPT). A preferred carload train may get 3 HPPT, a stack train 2.5-3, and a hot trailer train 4-5 HPPT, all over the same territory.
Drag freights and unit coal and grain trains are generally assigned based on published tonnage ratings, which as Balt said are just enough to get the train over the road.
Just to make it interesting not all locos are created equal for drag service. DC units have a minimum continuous speed which is the lowest speed that they can be operated under full power for a long time. Think SD-40 and variants and a MCS of about 12 MPH. AC units do not have the MCS limit, they can be operated all day at 2 MPH without damage. This is big reason carriers are careful about mixing AC and DC units in the same consist since crawling up the hill at 5 MPH will damage DC traction motors. Of course if a train has enough HP, it can get up the hill at 12 MPH and all is well, but that negates the idea of absolute minimum HP on drag freight.
BNSF operates 110 car export grain trains over Marias Pass MT 1% westward ruling grade. How much power do they need?
The grade is 20#/ton resistance and everything else is say 4 #/ton. Train weighs 110 X 143 or 15,730 tons. AT 24 #/ton need Tractive Effort of AT LEAST 377,000 pounds. With SD-40 at 90,000# need 4.18, actually 5 units. With modern AC at say 142,000# TE I need 2.65 so I assign three and take the excess in speed greater than what the units were rated at. Couplers are all grade E which are rated for 360,000# IIRC. The power can put out more than that but train resistance should not be more than 377,000#. All involved hope there are no bad knuckles in the head end or the crew will find it and tie up the line for an hour or so, in good weather, to fix it.
In practice the units will be rated at 5900 tons. Roundhouse foremen, dispatchers and crew will find they have capacity of 17,700 tons and a train of 15,730 tons so they are good to go. Of course since they do this day in and day out no need to really think about it until or unless the railroad is power short or a unit dies or some other disturbance occurs.
Actual units are usually selected by Roundhouse Foreman based on expected tonnage from yardmaster, dispatcher, or centralized power desk. Practice varies by railroad, by locality, and by time in history.
What the computer spits out is load, empties, tons, and feet based on what the clerk shows on the train which is based on what yardmaster and switch foremen told the clerk. Train will be scanned out of the yard which should correct consist errors.
PS at 8:32 PM
IIRC BNSF is actually operating DPU mode, 2 on the point and one on rear, to limit draft forces in head part of the train.
Thanks for your thorough and readable explanation.
CSXT's tonnage ratings are based on locomotives having an adequate flow of traction sand. The specification is 12 ounces per minute per sander. Typical reductions in flow don't generally cause stalls; however the combined affect of a significant flow reduction, poor rail conditions, and a steep grade can prevent a locomotive from moving its rated tonnage.
Mac's got it.
About the only thing I'd is that some trains add and drop along the way, but the power is usually planned to go the whole route. Most RRs have central offices that take care of monitoring this, power balance issues and routing units to shop for inspection/repair. Some even have GUI software that helps in the power assignment - just drag and drop the power onto the train.
-Don (Random stuff, mostly about trains - what else? http://blerfblog.blogspot.com/)
PNWRMNM
I think of my self at least as qualified as any other....42 year (retired in '02) rail. Your 6:16 PM explanation is flawless, clearly stated and involved.
Thank you. Have a good trip!
What an interesting discussion. Although I spent most of my working life as an accountant, I have a reasonably strong math background, and I can follow the calculations. Two questions:
Who makes the decision regarding how much power to assign to a train?
Would I be correct in assuming that the initial pass as to the amount of required power is spit out by a computer, which is reviewed by the person(s) responsible to assigning the power, and adjusted as necessary?
OK, now that we've settled all this, what about weather? (Always glad to add a kink to a discussion )
Hump yard computers, even in my day (early 70s), took journal temperature into account when calculating the retardation required for each car. I seem to recall a Davis variation that had an adjustment for temperature and maybe humidity but the details are hazy (no pun intended). But I clearly remember assigning extra power when snow was in the forecast.
Of course, on the Central in those days, it was unthinkable to assign only as much power as the tonnage required unless it was a low priority train. SVs, for example, always had four, five, or six of the newest GP40s available. Nobody ever got fired for overpowering an SV but a late SV could get somebody terminated.
ChuckAllen, TX
daveklepper Should not the values in th modified Davis formula be modified depending on revular journals or roller bearings?
Should not the values in th modified Davis formula be modified depending on revular journals or roller bearings?
No.
A well designed plain bearing only has a higher coefficient of friction when first starting. Higher torque is needed only to break the the oil film. Once running, the oil film becomes hydrodynamic in nature, and the friction is similar to a roller bearing.
The main advantage of a roller bearing on a rail car is the reduced maintenance vs. a plain bearing.
Tonnage ratings my carrier uses are based upon how much tonnage the locomotive can keep moving just above its Minimum Continuous Speed on whatever the ruling grade is.
Personally, I take exception to this, because like it or not a train with max tonnage for it's engine consist will have some form of mechanical trouble and stop on the ruling grade for the territory. While the locomotive consist may be able to keep the max tonnage moving - there is not enough power to START the train on the grade. To get the train moving again, assistance will have to be secured. Assigning tonnage rating based upon MCS is a gamble, the gamble being whether the train stops on the ruling grade or not.
Never too old to have a happy childhood!
As to Cuesta I suspect the difference is due to grade E couplers and knuckles rather than grade C.
Found 2 cents, here they are....
For DC locomotives, empirically determined, but based on calculations, a territory is determined to need a supply of horse power per ton of trailing load. The territory may include momentum grades, where an over-tonnage train may not be able to start but in a continuous train movement the train will lose speed but not stall before getting over the grade's summit. The phrase "making a run for the hill," comes to mind.
Hp/T ratings average out all the results of the more intricate calculations, and may include or exclude local factors like the mile of 1 per cent at the Martinez Bridge on the way from Oakland to Roseville, where exploiting short time amperage ratings allowed 3-tenths of a horse power per ton to let me get 5800 tons over the 'road.
Where Windy Point, near Palm Springs, and Midway on SP's Altamont trackage made tonnage ratings based on the formulas seem fictional, Hp/T usually worked. Not always.....most of the trips.
Many formulaic conclusions may never fail. Hp/T may never fail but also...as the weight of cars increases the effects of train resistance diminishes.
On Cuesta, the grade from San Luis Obispo toward San Francisco, I fired trains that were limited to 4250 tons with only power on the point.
Now, on the point, 5400 tons is the limit....roller bearings....no solid bearings....heavier cars...with less surface area for wind to affect...
timz BamaCSX83 if I could only find a CSX tonnage ratings book for the area I'm in (CSX Dothan Subdivision), I think I'd be set...Best not to trust "tonnage ratings" that much. Some RRs try to be realistic and some don't bother; none of us here is much of an expert on which is which.
BamaCSX83 if I could only find a CSX tonnage ratings book for the area I'm in (CSX Dothan Subdivision), I think I'd be set...
What I posted is what is used for the 'Bow Line' from Waycross to Birmingham that includes the Dothan Sub
And even that can be 'fudged', by setting policy that trains in one direction must fit the sidings and trains in the other direction can exceed siding length. Makes it a little more difficult for the Train Dispatcher but it is a workable strategy.
BamaCSX83 Now, if I could only find a CSX tonnage ratings book for the area I'm in (CSX Dothan Subdivision), I think I'd be set...
Now, if I could only find a CSX tonnage ratings book for the area I'm in (CSX Dothan Subdivision), I think I'd be set...
DEPART - ARRIVE - SD402 SD50 C40-8 ES44D SD70MA CW44C CW44AH NASHVL - BIRMIN - 2650 3150 3400 3700 4350 4600 5000 BIRMIN - MONTGL - 4200 5000 5450 5850 6900 7350 7950 MONTGL - DOTHAN - 2800 3300 3600 3900 4600 4900 5300 DOTHAN - WAYCRO - 4050 4800 5250 5650 6650 7050 7650
WAYCRO - DOTHAN - 3250 3850 4200 4550 5350 5650 6150 DOTHAN - MONTGL - 2850 3350 3700 3950 4700 4950 5400 MONTGL - BIRMIN - 3350 3950 4350 4650 5500 5850 6350
Forum formating doesn't provide accuracy in tabular data.
Mac, you're right. I've heard those exact things broadcast on various scanner feeds when a train is coming out, or heading into a yard. But with those tonnage limits, (and some rough calculations/guesstimates) I could more, or less, come up with a max # of cars based on the power on the head end.
As others have mentioned real world ratings are in tons between two points, not cars. For each train there will be a consist that will include somewhere on it loads, empties, tons, and feet. These are the important numbers for train dispatching.
Thanks to all for the answers, I believe me, I know that straight and level track is NEVER straight and level...
And again, I have seen a single locomotive start a cut of cars that seems "impossible" for it to move, but has indeed, moved it;, but I have always wondered how the "powers that be" have determined exactly what each locomotive is "rated" for as far as car numbers. Now I've got something to not only work with for grins and giggles, but gives me a better idea of what the "real guys" do.
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