You can take the jitters out of it by adding angular drag. Add enough and the train will just sit still. You’ll need to add more force to make it move again of course and will take some playing around to get the right balance. In fact, you might end up with a much more train like feel if you make it nice and draggy.
EDIT: I do agree with the guys above saying that physics might not be the best idea. But if you intend on crashing the train or want hills and things to affect it you’ll want to stick with physics.