Have been seeing more BNSF stack trains with 2 head 2 middle and 2 pusher DP locomotives lately on the Chillicothe Sub Coal City Il. Union Pacific was the regular DP on the sub before. Has BNSF expanded the use of DP on mainlines ?
Do those stack trains seem longer?? With its major Chicago logistic center for stack trains just a few miles from Coal City and with deminished business elsewhere perhaps BNSF is utilizing available extra power to build longer trains and create faster end to end operations.
Standard cold-weather operations. Power spread throughout the train not only aids in general handling over steep terrain but also (and more importantly in the case of high-priority intermodals) reduces the time needed to release/charge the air, which moves more slowly through the pipe in frigid winter temperatures. The more places you put locomotives in a train, the more places you have compressors pumping air into the brake pipe. Despite some recent increases in daytime high temps along BNSF’s northern and central territory, nighttime lows are still well below freezing:
http://www.intellicast.com/National/Temperature/LowTomorrow.aspx
A year or two ago, Z and Q trains on the South Seattle-Chicago and Portland-Chicago lanes were typically adding a single DPU to the rear during winter. This winter, I’ve been seeing those trains crossing the Idaho panhandle with three to four units on the head end and two DPUs on the rear.
Is that last claim actually true? Everything I’ve ever heard indicates that the increased time to charge is either because the seals in the various connections ‘leak’ more in cold weather, or there might be some condensation reducing part of the effective diameter (which cuts down volume, not makes the air ‘move more slowly’ as if it were a viscous fluid, and thereby produces a slower pressure rise on the other side of the obstruction).
Can’t really speak from any other resources but my own observations, but here in South Central, Ks. What I have noticed is more of the use of single DPU engines at the rear of some trains( unit grain moves, some mixed merchandise moves and unit tank trains).
Previously, there were usually two rear-end DPU’s when there were rear-end DPU’s used. It seemed to be a BNSF policy (?) BNSF always seemed to use a pair engines on rear DPU’s; while UPR, it seemed would used only a single DPU on rear DPU positions.
What I have noticed is a pair of trains (Eastbound and Westbound) that come through here ( Westend of the Emporia-sub where it junctions towards Wellington) that use mid-train DPUs. These trains are generally all Export containers in double stacks. Sometimes the second half of the train will be a mix of Domestic cans, and TOFC trailers. Eastbound there seem to be more, and more refrigerated trailers(TOFC) with the units running. That becomes pretty evident when they pass at night with their units on, and the indicator lights flashing on/off on the unit panels.
The last claim is true. Air does move much slower thru brake pipe in colder temperatures. DP enables faster charging during cold weather. CN actually has built air compressor cars in intermodal well cars for freezing weather to help.
Air at 0F is about 87% the volume of air at 70F so it takes more to make the pressure. Kinda like how you have to add air to your tires in the winter.
A follow-on to the boxcars so configured that they’ve used in the past.
Doesn’t move slower - just leaks more so you think it is moving slower.
Isn’t colder air slower due to the fact that the speed of sound is directly related to air temperature. Believe Mach 1 is ~ 746MPH at 59 degrees F. At -40 c ( or f ) much slower but cannot remember exactly.
You better go back and look at a reference. Colder air is denser (translation of the physical meaning of the previous ‘volume is less’ post). Speed of sound is higher in denser media, so the speed of propagation of control impulse in a trainline would be quicker, not “slower”
What he’s talking about is physical flow speed, not shock propagation, though. Air is a gas, so what you want is the change in effective viscosity associated with the higher density – and yes, technically the denser air would have a higher viscosity. My point was that I did not expect this to produce substantially lower physical flow speed for the air passing through the line.
Denser air in the trainline also involves more volume to be pumped (see the previous point about tires in winter). This might require ‘more’ compressor run time, but the intake air is also denser so less work needs to be done to compress it, and less compression heat then removed, and probably less condensed humidity that has to be blown down with air pressure in traps. I’d be thinking proportional…
As Balt noted - it takes longer to pump the train up. Gaskets get cold and stiff so the system leaks just that much more. Pumping up a big train that’s been completely off air can take an hour or so at “normal” temperatures. If it’s cold - that becomes hours.
A lot of the moisture ends up in the main reservoirs on the locomotives - I’ve certainly drained enough water out of them.
Modern locomotives have air dryers and spitter valves in a effort to purge moisture from the trainline air supply. By rule, alcohol that in days gone by was used to keep trainlines from freezing is prohibited.
Well, there you go - I run antiques… Although the spitter valves on our “newer” ALCOs drive me nuts…
Part of the problem here is that almost all the water in the air going into the compressors winds up in the tank, while the ‘leaks’ blow out a good part of the air that was compressed. So if it takes you an hour to bring the line up to pressure, you’ve pumped far more of a volume of vapor … which at some point has to be let out or expelled. The ‘good’ part is that the condensate may be relatively hot at the trap/valve, precluding it from ‘cold effects’ if its ‘action’ gets too slow…
It’s not unusual on the very cold nights to watch pressure on the EOT gradually drop 5 or 6 psi while the flow goes up. The drop is slow enough that brakes don’t set up. Then once the sun starts warming things up, you see the opposite. Flow starts going down and the pressure on the EOT goes back up to where it was in the beginning.
Jeff
All you want to know about the speed of sound and compression waves. Speed in air is constant times the square root of the absolute temperature.