I was wondering, did any railroads couple 7 or 8 locomotives together on, lets say… a unit coal train coming down a VERY steep grade. The electrical cable is cut off at the 4th loco so when the engineer needs power, only the 4 locomotives respond. But when he needs brakes, he will have the braking power of all 8 locomotives because you obviously wouldnt cut off the brake pipe feed. Would it work? Does anybody use it? Tell me if its stupid. PLEASE!!
I’m not sure I understand the point of this.
You’d want dynamic on as many axles as possible to control the train. That would indicate that every locomotive’s MU control would be active to the extent of providing properly-modulated dynamic braking… which would also imply control over engine power. Meanwhile, you would NOT want to make any brakepipe reduction until safety considerations made it necessary, and you’d also want to keep any air-brake application as minimal as possible (wheel wear as well as brake wear, heat, etc.)
Is your question more directed toward “can an engineer restrict the number of units in a consist that respond to throttle-up, in order to save fuel, while maintaining full dynamic-brake actuation on all units”? Perhaps Randy or others with distinctive competence can indicate if something like Select-a-Power works this way…
Yeah, thats what i’m talking about.
For an indepth look at how loss of braking power can occur and it’s results the following NTSB Investigation is illuminating.
You don’t engage too many d/b’s (especially extended range d/b’s) unless you want to run the rail out from under the train like so-much spaghetti. You’re gonna come face to face with one roadmaster and a road foreman of engines that are going to want your head on a stick and your butt nailed to the floor.
The locomotive engineers on here will tell you there is a direct calculation of tonnage, d/b’s and wheels/axles to be made before you take off with that train. If you do not and you spill the train all over the countryside, you are in search of a new vocation.
Have recently seen on the Illinois & Midland where the combination of d/b’s and high adhesion trucks has literally walked bridge stringers (timber beam sections) off the top of the bents (piers) in the direction of the loaded coal trains…incredible friction forces on the rail involved here!
(for every action, there is an equal and opposite reaction…think about it!)
[banghead][banghead][banghead]
On mountain rail, even slow speed mountain rail, the rail always wants to go downhill. Mudchicken illustrated this just above. The force exerted by locomotives to lift a train up a hill will exert sufficient longitudinal force on the rail just as if you had grabbed ahold of the end of the rail and pulled it with a powerful locomotive. The same happens when a train goes downhill, but now the effert is magnified by the force of gravity and the use of the air brakes.
Apply too much longitudinal force to the rail and things that should not be moving, move to some strange places in novel ways. Rails break apart or joints pull apart at hill tops, rails “buckel” like a sunkink at the bottom. Stringers move off the bents on trestles.
Roadmasters “just love” this situation. One line, 8 miles of 1.8-3.3%, 5 SD9’s and 25 cars each way 3 times a week, and the RDM had to cut 1/4 inch of rail each 6 months out of JOINTED rail. It is even worse on CWR.
Good point, MC.
We think so much about the lateral forces and the downward forces, but even with a relatively low COF under a 500-ton or more locomotive, that’s a lot of force pushing that rail forwards or backwards. Since there is a lot less resisting that force on the rail, I can see many opportunities for mischief.
I suspect in your wanderings you have seen more than one occurrence.
With the Select a power and with locotrol (DPU) you have alot of flexability to do things you couldn’t do with a straight MUd consist. Instead of having all your power on the head end, you can spread it out throughout the length of the train. On many railroads you are limited to 200 k lbs in draft and 400 k lbs in buff ,with most locomotives a large consist can easily exceed this.
We cannot use dynamic brakes here currently simply because the track is not well anchored.
Randy
You don’t want too much braking force at the head end or you’ll wind up derailing cars near the head end when on curves. A rule of thumb limit is 24 powered axles in braking is the max. When this number of axles is exceeded in the consist, the DB on the trailing units has to be cut out.
So, in practice, you can have the exact opposite of what you suggest.
and all of the comments above in one way or another illustrate why distributed power is, to put it mildly, popular!
One thing that you learn as a engineer. All the rules and the facts above are fine. we run up and down the divisions all the time with tons of power (3-4 ac’s) and fairly large trains. Just don’t mess anything up and everything will be fine its when you tear a train up or jacknife one that the ***** hits the fan and the rf of e wants to talk
A SD40-2 is rated at 6 axles for braking. An AC4400 is rated at 12 axles. So, by the rules, two AC4400s would be the equivalent of 4 SD40-2s in braking, or in power. Excess DB axles would have to be cut out. There is a switch in the cab to do this. Newer power have switches that cut out DB either due to defect or due to having too much DB available.