The problem is that if your ‘locomotive’ stops; all the layers would come crashing into it. 🙁
The easiest solution that springs to mind would be to precompose all your layers using the ‘follow the leader expression’, do the motion of the loco in constant speed, that way all the wagons will keep the right distance from each other.
Then use time-remapping on the pre-comp for slowing down, reversing, stopping etc.
But perhaps there’s still a more logical approach that would let you just animate the loco-layer.