Engines pushing

[edblysard]

Murphy,

Because when you release the brakes, they start releasing from the head end backwards to the rear...it is possible to have the head end with the released brakes moving, and the rear of the train sitting still with brakes applied.

When this happens, you waste fuel and time pulling against the brakes.

With a DPU set up, the brakes release from both ends, the release meeting in the middle, plus the rear unit pushing the rear of the train, with its brakes released, forward at the same time the head end is pulling.

Not a huge difference, but it adds up.

Less fuel wasted pulling cars with the brakes set, less time accelerating.

And the add further to Mr. Husman's explanation...the Train Link set up was not standard equipment on most locomotives...if you wanted it, you either ordered it as a "option" or had it retro- fitted.

Now days, most locomotives come with it...all the SD70Ms I have been on have the DPU set up as part of the engineers console and the computer screen.

All the new GEs have it...it comes as part of the package.

But look back at say, the Santa Fe snoot nosed SD40s...the extra long nose was added to house the then new equipment...if you find a SD40-2 with a Train Link set up, it is a add on unit, mounted above the radio usually...one of the reasons you are seeing more DPUs is because railroads finally have enough locomotives with the DPU capability to use it effectively on most trains, not just the unit or bulk trains.

Technology simply caught up with the railroads, or the other way around if you think about it!

 a. more capacity on line,faster starts n stops, easier to maintain speeds,better braking.        

  I still don't comprehend how the train will start faster.  Can you explain please?

[Modelcar]

...Simple questions:

Why does the DP train accelerate faster from a stop {after brakes are released}, than the train with {the same amount of total hp engines all located in front.....} ?

What is the logic that allows a non DP train stall on a grade compared to same weight train with DP making it up same grade without stalling...?  I'm not doubting folks word that have done the exact two runs....but what is the logic that allows this....?

I have no doubt a train with DP can be made to handle better.  I simply thought this was a dicussion on one getting better fuel economy than the other...{Especially where "handling" conditions are not an issue {for our measurement here in comparison}.......In other words getting more out of available hp with one than the other.  {Example:  The two trains climbing the grade and stall or not to stall...}

[edblysard]

Q,

First, your qualifier..."after the brakes release"...that is part of the whole thing...with a DPU, the brakes release from both ends and meet in the middle, the rear unit is already moving the rear of the train forward, so you already have acceleration, instead of having the head end pull against the brakes that have not released yet.

You don't wait for the all the brakes to release before you throttle up, the slack on a grade would pull you backward, and then you have to overcome that...plus pulling on the brakes as they release backwards to the rear eases the slack out in a controlled manner instead of a sudden "snatch" or jerk against dead weight, lots better for the couplers.

We have been getting a lot of grain trains in here with DPU, and trust me, when the train has a unit on the rear, you hardly feel any slack on start up...the engineer knocks the brakes off and throttles up, and the whole train moves almost at once.

 

As for you second part, the stall on a grade.

Keep in mind it is the weight and tractive effort of the locomotive that moves the train...with all the weight and traction on the head end, it is possible, as the slack runs out, for a train of sufficient weight to pull a train backwards down the grade...but put a locomotive with all that weight and tractive effort on the rear, and have it pushing on the rear...then the weight of the train would have to overcome the tractive effort of the locomotive on the rear and the tractive effort from the head end also.

[Modelcar]

.....Thanks Ed.....I'll have to let that soak in and see if I can make sense of it.

[beaulieu]

Another benefit of a DPU is that it reduces the curve drag. Imagine a long train strung out around a series of reverse curves. With a DPU there is a bit less of this. It's not a huge amount but it adds up over time. It's the cummulative amount of a lot of small benefits, not one big gain.

[Modelcar]

....If the "hill" or grade has a tangent track, then whether engines are front or set up as DP.....does one have any advantage getting the same hp on the track to keep the train moving and not stalling, to make the hill....?

Decades ago in our home area of Pennsylvania the B&O branch about a mile away with twisting grades always used "pusher" help in taking loaded trains up thru the valley.  Most of the time 2 engines up front and one {sometimes a larger one}, would be on the rear.  They would have to stop on the way at each mine spur and one engine cut off and go back to collect the day's worth of loads.  Why the pushers in rear.  Don't know....I wonder if coupler strength was part of the issue....?  Grades in the 2% bracket, and I believe one section touched 3% and along with a sharp curve called "cat head curve".....Have no idea where that name came from.  Also have no idea if the curve was compensated in grade {for it's curvature}.....

Edit:  In my own mind, I can see in track as described....the pusher would allow {help}, to keep part of the train "off the inside rail", and {at least by theory}, lessen the drag on the curves.

[timz]

it has been tested and is a done deal.

We assume it's a done deal, all right-- what we don't know is why. For all we know maybe there's a good reason, but none of the attempted explanations in this thread is close to convincing. (Reduced curve resistance sounds reasonable enough, but people are claiming the DPU train rolls easier even on straight track-- aren't they?)

Part of the reason it wasn't done sooner is that it took years of data from acutally running trains to prove the point.

Why would it take more than a couple days? Pull a train from A to B with three units on the head end, then return to A and repeat the run with the exact same train with one unit on the rear. Repeat as necessary to get a good average.

The non-DP train accelerates for 15 minutes (.25 hrs) before reaching
cruising speed....

The DP train accelerates for 10 minutes (.17 hrs) before reaching [the same?] cruising
speed ...

In other words, the DPU train rolls easier-- it has a lower rolling resistance at a given speed. Lots of people have claimed that, and for all we know it could be true-- but no one here knows why, and no one here has given any evidence.

[dehusman]

In other words, the DPU train rolls easier-- it has a lower rolling resistance at a given speed. Lots of people have claimed that, and for all we know it could be true-- but no one here knows why, and no one here has given any evidence.

Never said that.  It accelerates better.  It accelerates better because the brakes release faster and the rear engine starts the rear of the train moving the same time the head end starts moving.  The rolling resistance is the same, the ability to overcome the rolling resistance is better with DPU.

Considering that the actual studies and the actual "evidence" are proprietary information of the railroads, I rather doubt you will get any "evidence" other than anecdotal stories (which you seem to dismiss) and that the railroads are, in fact, DPU'ing the trains.

Dave H.

[oltmannd]

DPU has much more to do with drawbar strength than anything else.

Take the ruling grade, tonnage, and 20# per ton per %grade and find the needed TE for the grade.  If it's less than 250-300,000# (450,000# for unit coal trains), the all the power can go on the head end.  If not, then, DPU.

I don't think the reasons for using DPU go much beyond this.

[timz]

It accelerates better because the brakes release faster and the rear engine starts the rear of the train moving the same time the head end starts moving.

So as long as the brakes on both trains are all released, they accelerate the same?

I rather doubt you will get any "evidence" other than anecdotal stories (which you seem to dismiss)

No one in this thread has tried to give any quantitative evidence of an actual train's performance. Nothing to dismiss, so far.

[JayPotter]

I'm unclear about what issue we're discussing.  Is it (1) the difference between using one, or more than one, locomotive consist on a given train or (2) assuming that more than one locomotive consist is used, the difference between the second consist being a manned helper or being an unmanned distributed-power consist?

[oltmannd]

I'm unclear about what issue we're discussing.  Is it (1) the difference between using one, or more than one, locomotive consist on a given train or (2) assuming that more than one locomotive consist is used, the difference between the second consist being a manned helper or being an unmanned distributed-power consist?

Jay-

It started out as "why are RRs using DPU more often" and it segued into "using DPUs saves fuel", "does not!", "does, too!'. etc.

It seems that we need a theory and some experimental evidence that would explain any fuel savings in order to stop (or slow down) the bickering. 

Got any?

[JayPotter]

It seems that we need a theory and some experimental evidence that would explain any fuel savings in order to stop (or slow down) the bickering. 

Got any?

Not really.  According to GE, its DP equipment should produce a 12% average increase in traffic movement with an average operating cost reduction of 10%; and an unquantified fuel-savings factor is included somewhere in there.

The only DP application with which I'm the least familiar is CSXT's.  I have the impression that their initial DP objective is to eliminate manned helpers from helper districts but that this is only cost effective if the DP-equipped trains are significantly heavier than the trains that had been shoved by the manned helpers.  In other words, the ability of a DP-equipped train to generate increased ton-miles compensates for the increased cost of having a DP helper consist remain on the train for a longer distance than the manned helper would have remained on the train.

It had occurred to me that your involvement with Conrail's SD80MACs might have given you some insight into Conrail's DP expectations.   I know nothing about Conrail; although shortly after the Split, CSXT did test SD80MAC consists in DP service.  My understanding is that in order to justify the increased cost of what were essentially long-distance helpers the DP-equipped trains had to be so long that they reduced the fluidity of the route.

It wouldn't surprise me if the DP-or-no-DP issue is largely a matter of increased ton-mile revenue versus increased horsepower-hour costs.  I have a feeling that the presence of a DP consist -- or, for that matter, a manned-helper consist -- can reduce fuel consumption to some extent; however I expect that the increase in ton-miles is a more significant factor than the reduction in fuel consumption.  At least that's what I expect with regard to CSXT.  I have no idea about western roads.

 

 

 

 

[UPRR engineer]

DPU has much more to do with drawbar strength than anything else.

Take the ruling grade, tonnage, and 20# per ton per %grade and find the needed TE for the grade.  If it's less than 250-300,000# (450,000# for unit coal trains), the all the power can go on the head end.  If not, then, DPU.

I don't think the reasons for using DPU go much beyond this.

There really is more to DPU then that, it helps out everything to do with a train. Since it does save fuel, that is the biggest reason you see the big push now. I have a friend in Omaha, he is one of the guys who crunches all the numbers and looks at the fuel spent while looking at the locomotive downloads. Loaded coal train, average fuel saving on the trip was 800 gallons of fuel saved.

There is a Darkside to all this... Anyone able to figure it out???

 

 

[UPRR engineer]

.....Quicker starts and some of the other thoughts may have possiblities....I don't have the expertise or facts in railroading to know such.  But I do know one thing...Jack rabbit starts in our motor vehicles wastes fuel.

Nope, we're not suppost to goose it off the line eather there buddy. Running to save fuel is #1. Union Pacific uses more diesel then the US navy so id guess that puts us as the highest user in America, thats not something to brag about, its the reason we (supposed to) take every measure to conserve as much as possible. The good old days of running like ya stole it are long gone.

 

Someone asked about DP on the Z piggyback trains, you might have to wait till the railroad (or the customers) decides there gonna set down on the service we provide. Something like instead of running it daily they decide to ship every other day and make one train out of two. Once it becomes less time sensitive and they just want there trailers to make it there. Money and oil, once that becomes the deciding factor, then you'll see it.

[Modelcar]

....All the comments from the folks in the know...{railroad operations and operators}, has been interesting and some things surprising.

It just is difficult to see same hp whether at the front or DP can make a difference in how much fuel it takes to move a certain weight from point "A" to point "B".  I wonder if we had a test track such as the test facility out west where "numbers" could be layed down in a direct comparison it would show a difference to measure....?  And if it does...Why....?

[oltmannd]

Conrail borrowed some Locotrol C36-7s from ATSF and tried them out on some coal trains out of the Mon Valley.  The goal was to try to push the train size up.  There was some opportunity to run longer trains up to Ashtabula and over to Enola.  If I remember right, quite a bit of Conrail's 100 ton hopper car fleet was built with grade C draft gear which limited the train size much more than line capacity.  I think it was something like 90 cars to the east (if you didn't push all the way from Pittsburgh and 110 cars to the north.  You could do 135 cars to the east if you had grade E draft gear.  Most of the private equipment operated out of there was grade E. In fact, the SD60MACs tested on a 135 car coal train from W. Brownsville to Enola.

I think this was done in concert with opening Shire Oaks and downgrading West Brownsville as the origin point for these trains.  It was easier to put together the DPU trains at Shire Oaks.

That led to putting Locotrol on 10 C40-8s and purchasing the equipment on the SD80MACs, although the SD80MACs were never intended to be used solely in unit train service. 

I don't recall Conrail ever really settling in to using the Locotrol.  I think it was such a pain in the neck to the link set up, it wasn't worth the effort, and keeping the equipped locomotives together was hard, too.

Conrail did try some "stupid train tricks".  They ran some doubled-up ore trains to Philly using the SD80MACs DPU capability.  It think they were hard to deal with on the approach to Philly and at the terminal.  They also tried running some doubled-up Selkirk to Elkhart trains, but scattered one all over the scenery once, and that was the last of that.

Even during the SD60MAC demo period, there was some questionable thinking.  We had laid out the rules for the demo period.  A bunch of the rules included contingencies for the AC propulsion messing with the cab signal and grade crossing circuits, but one rule was about not putting all three of them on line when pulling a regular old freight train up a grade.  Well, there was a lot of interest in seeing if the AC traction could eliminate the need for helpers.  And, despite our warnings, the road foreman one night decided to "just see" if he could get this particular doubled-up empty ore train over the Horseshoe Curve with out helpers by putting the 3rd unit on line.  Our guy on the train told him, "no, you'll break the train,  450,000# TE is too much for grade C draft gear".  He did it anyway, and broke the train.  Oh, well.

Then, after the SD80MACs started being delivered, the Transp. Dept came out with a plan to run them all over the RR in all kinds of service.  They were really a poor fit for intermodal trains or flat-land merchandise trains.  It was a waste of the investment in AC propulsion.  What we really needed to find out, in short order, was whether they could really do 35% adhesion, day in and day out.  We tried to get the plan cut back to more suitable service and routes, but to little avail.  They finally wound up on the Boston Line, which wasn't a bad fit except the train density and scheduling kept the utilization pretty low, which was a waste of the investment, too.

So, I am very skeptical about claims for fuel savings with DPU.  I don't see any theoretical case.  The energy needed to move a train from A to B is still governed by physics as expressed in the Davis equation.  HP/ton is HP/ton no matter how it's placed in the train.  DPU may do lots of good things for train handling, etc, but I don't see where the fuel savings could come from.

If Conrail was typical, there is generally a gap between facts and RR managment folklore.

[JayPotter]

There is a Darkside to all this... Anyone able to figure it out?

I really don't know; but it wouldn't surprise me if it related to the fact that the DP units are unmanned. 

At least on CSXT, unmanned DP units are used across entire routes on trains that are longer than they would have been if they had only head-end power across the entire route and were assisted by manned helpers across helper districts.  This apparently works well unless a DP unit experiences a failure.  If this occurs on a segment of the route on which the DP unit is necessary to keep the train moving, the train stops; and someone from the head end has to walk the entire length of the (longer than it would have been without DP) train to investigate and try to correct the failure.

[rrnut282]

Instead of stating trains with DPU accelerate faster, which is physically improbable, why not state that DPU trains can accelerate sooner because the brakes are released faster?  Instead of waiting for a good release before notching out the throttle, an engineer can apply more power sooner which lessens overall time to track speed.  With the brakes applied from both ends of the train, stopping distance is reduced, meaning a train can stay at speed longer, which will shorten overall time across the division.

Another benefit:  in hog-back territory, does an engineer apply brakes or throttle when the back half of the train is coming downhill to keep slack stretched?  Wouldn't it be better to initiate a light brake application from the rear to slow the back half of the train and keep the slack from running in?  Then with the train stretched, you're ready to hit the next hill.  Or you could apply DPU power to keep the slack bunched in the back half of the train.  Either way the train is handled, there will be some fuel savings.  In the first scenario, the engineer avoided revving up the power, in the second only one loco is revved instead of two (or three or more).  DPU gives the engineer options in train handling that should result in smoother, overall faster transit times.

[Modelcar]

.....Now that makes more sense {from rrnut282}, than all the rest I've heard so far....Simple logic.

[Rodney Beck]

No rrnut282 I can operate the rear independent from the head, I can move to back to have the rear dp about 2 throttle notches above the head end. Everyone is right the brakes do release sooner as air is pumped from each end at the same time. The te/amps to start a train is reduced quite a bit.

 

Rodney

[UPRR engineer]

There is a Darkside to all this... Anyone able to figure it out?

I really don't know; but it wouldn't surprise me if it related to the fact that the DP units are unmanned. 

 

I think its gonna make hoghead only trains very easy. Having a GPS marker (the rear DP motor) will make positive train control a reality i think. Holding off shipments till they have enough cars to make a Super Train, or combo-ing two trains together farther down the line to reduce crew usage on the rest of the trip will also save them money with the way things are going with oil and the economy.  

[jeffhergert]

The Darkside is that when you need to go to the 2nd unit to use the "facilities" and that unit is 100 cars back.

Just got called for a 1 and 1 manifest.

Jeff

[dima]

I am wondering about slack within the train when running with DPU on the rear. How does engineer control it? When all locomotives are on a head end the train could be either stretched or bunched. But when DPU is in the rear then surely in one particular place within the train there will be a "change" from stretched to bunched. How is it affecting couplers for the particular car that is on this "border"?

[UPRR engineer]

Yep there is always some slack in the train, you just have to think about where your at before doing something. Theres many different options to how you want to run the train. Exp. If you running over some whoops, you can have the lead in dynamics and the rear pushing you, slack stays bunched up. Most of the time you can just run it like you would run any other train in sync. mode, (most of the time) as long as you dont make any aggressive moves with the controls it has the same feel. Hope that helps.

[rrnut282]

No rrnut282 I can operate the rear independent from the head, I can move to back to have the rear dp about 2 throttle notches above the head end. Everyone is right the brakes do release sooner as air is pumped from each end at the same time. The te/amps to start a train is reduced quite a bit.

Rodney

I knew that, I just wanted to keep my example simple with only one variable.  Even so, you're still making my point of not having all 3 units (if they were up front and M.U'd) at a higher notch.

[ogre427]

Just a couple quick thoughts that come to mind reading this thread. First, as to the comments that this is a new thing, that's not true. The BN tried using mid train units with "locotrol" systems in the mid 70s. While they ran them for several years, it was ultimately the technology of the day that let them down. Radio communications and electronic controls (neither one of them digital at the time) just weren't reliable enough. Since then there had been an ongoing search for a way to move units off the head end of trains. Only lately has the technology arrived that is up to the challange. The biggest advantage of the system is contained in the name: "distributive power". I.E. having your power distributed through the train instead of at just one end. Just imagine the forces exerted on that first coal car as they pull 125 loads up a hill using head end power only. Now imagine if part of the train is being pushed from the back instead and you can see how this would save wear and tear on equipment. The wagon comparison is a good one if you are pulling the wagon with a thin rope. There is a limit to how hard you can pull before the rope breaks, but if part of the force is pushing from the rear you can move more weight without the rope breaking. That alone is worth using DP, and IF it saves fuel, well that's just gravy. As for the location of the locos in the trains, somewhere in the middle (about 2/3 to 3/4 of the way back usually) would be ideal because then they would be pushing part of the train and pulling the rest. Unfortunatly the logistics of getting locos in and out of the middle of a train are somewhat difficult. Say for instance you need to take the mid train locos out for service. First you have to stop the train at a turnout, uncouple from the locomotive, pull the front of train ahead a safe distance, and pull the locos through the turnout. All the while the back of the train is sitting without power, blocking the track. The problem only gets worse if the loco is dead. Compare this to simply coupling up to the back of the train, and the compomise of a less than ideal location is well worth the added convience. As for why the "sudden" change, DP equipment isn't cheap and maybe it is just now, as they say, "in the budget". (however this is just a guess)  

[Modelcar]

.....I witnessed a Sante Fe train blasting up thru Kingman, Az. in 1970 with 9 engines on the front and 3 more "cut in" back in the train but have no idea if they were manned or not....Don't remember that.  I was so shocked to see that many engines I don't remember looking for a crew in the engines back in the train.  They were east bound.