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General Electrics boasts that its Evolution series diesel-electric locomotives are capable of dynamic braking. Dynamic braking uses the electrical traction motors on the locomotive axles - motors are switched into generator mode and convert kinetic energy of the locomotive into electricity which is then directed either into large resistors (most common) or into charging batteries.

Sounds good but a typical scenario is having two or three diesel-electric locomotives pulling a train of one hundred carriages. A typical locomotive has up to ten axles and weighs up to two hundred tons, so three locomotives have up to thirty axles and weigh up to six hundred tons. They together pull around one hundred carriages and each of those has four axles without any motors (so unable to implement dynamic braking) and weighs around one hundred tons each (when fully loaded). One hundred carriages together weigh about ten thousand tons total - about 16 times more than the locomotives.

So it looks like if the locomotives use dynamic braking and the carriages don't use any braking at the same time then the whole train should "compress" and either derail or collapse into the locomotives because of inertia. So the carriages should use air brakes each time when the locomotives use any brakes so that the whole train slows down evenly.

This means that dynamic braking can only be used to slow down the locomotives, not the entire train and the carriages have to use air brakes every time the locomotives use any brakes.

Can dynamic braking be used to slow down the entire train or does it only slow down the locomotives?

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  • $\begingroup$ If it only slows the locomotives then there must be flexible connections... $\endgroup$ – Solar Mike Feb 6 '20 at 13:53
  • $\begingroup$ What aspect of dynamic braking means the cars are not using the onboard brakes? Is that explicitly stated? $\endgroup$ – fred_dot_u Feb 6 '20 at 15:47
  • $\begingroup$ @fred_dot_u The typical freight carriages have no motors. $\endgroup$ – sharptooth Feb 6 '20 at 15:58
  • $\begingroup$ "So it looks like if the locomotives use dynamic braking and the carriages don't use any braking at the same time..." Why presume no braking on the carriages? $\endgroup$ – fred_dot_u Feb 6 '20 at 16:00
  • $\begingroup$ @fred_dot_u There's "if" in there. $\endgroup$ – sharptooth Feb 6 '20 at 16:12
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Dynamic braking is used in combination with the air brakes in the rest of the train. As such it is capable of only capturing a portion of the kinetic energy that needs to be dissipated in order to stop the entire train.

For carefully-applied braking, the "jack-knifing" collapse of a "compressed" railway car string is avoidable.

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