In the recent past (with in the last year or so) I have noticed a few more UP trains running with an engine or engines pushing on the rear of the train. Now, in big grades, I understand, that practice is as old as railroads itself… But a few months ago, I saw train coming into Phoeinix and had a psuher on the back. (DPU?) That really surprsied me… well, it did unitl I have seen a few more. So, what are the adavantages to that kinda of power setup?
In theroy, the horsepower/weight ratio is the same, but is it easier to stop and start up a long train with rear power pushing as well? Less coupler stress? Any input woudl be great… Something I am trying to figure out.
one reson is trailing tonnage… if the tonnage behind an empty long car or what ever the railroad equipment resrictions are… the stress on that car could cause it to derail…also… over the terriorty… the power to ton ratio might be good for most of the run…but thier might be a grade that it isnt good for…so insted of running to small trains… it is just as good to run 1 bigger train and just add extra power if needed…and with DPUs thier isnt even the extra cost of another crewman to have to pay to run the helper…
csx engineer
Ease of adding and subtracting power to handle particular grades where the pushe is not required for most of the route the train travels is another advantage/
With DP units, a brake application is, or can be, initiated from both ends at the same time, getting the brakes applied in half the time. This allows longer consists to be run more safely.
The main reason to use pushers is how hard you can pull on the train before you break it. Freight cars are designed to withstand a certain maximum pulling force. The components are designed to fail in a “graceful” manner. That is the center sill is stronger than the draft gear and coupler and the draft gear and coupler are stronger than the coupler knuckle. The reason the knuckle is the weakest is that a broken knuckle has few consequences and is easire to repair than a draft gear pulled out of the end of the car. Similarly, a failed coupler or draft gear has a much lower chance of a catatrophic derailment than a failed center sill.
A regular freight car knuckle is designed for roughly 250,000 lbs and a high strength one, typically used in unit coal trains, is designed for 400,000 lbs.
If you have 3 AC units on the head end, operating in notch 8 at 5-10 mph, they will generate about 450,000 lbs force which risks breaking a knuckle. If you put one unit on the rear, then the force on the head end is reduced to 300,000.
Each RR operating district seems to have individual challenges and physics and philosophys function in those frames of reference. The UP in this area (Southeast Kansas) on the old MKT southbound uses two units of higher HP ratings on the head end and a pusher in DPU mode at the rear, no major hills but a rolling terrain. To the east on the KCS southbound they use midtrain helpers in DPU as well as a rear pusher in DPU, their terrain south of Pittsburg, Ks is very hilly and mountainous. Thus the extra power. The BNSF on trains over the KC to Ft Scott and Monett to Springfield many times will use double pushers at the end of train position in DPU. You see this on their run through trains on the UP and on the FT. Scott sub. The KCS places their mid train helpers at Pittsburg and also removes extra units there also, to turn that power back south as helpers.
Sam
Seen this a few times out here, like gravel unit trains with an engine each end, and stack trains, with 2 engines each end, can’t tell if their comin’ or goin’.
Not only applied quicker, but also released quicker.
Another advantage is better slack control. There are situations where an engineer might be in hard dynamic with the head-end locomotive(s), and the rear loco still in power, just to control the slack.
In addition, as oltmannd alluded to, there is a practical limit as to how much strain you can put on a drawbar/knuckle before it will fail. Especially in hilly terrain, and especially just after the power has crested a hill and is starting downgrade, but the rest of the train is still climbing the hill. Once the power starts downhill, the pull of gravity is added to the tractive effort (visualize a string with a weight on one end, and you pulling the other end of the strng over the sharp edge of a knife).