basically if i understand right you want to have a carriage that is pulled by a train/bombcart
what you do is make a 2nd track train, and use the 1st one to control it's starts/stops (i.e. when the 1st one isn't moving, it sets speed on the 2nd one to 0.
have the carriage train use the same path nodes as the "pulling" train.
the only other thing you ahve to do is make sure that the carriage has a track path, the correct distance behind the start point of the "pulling" train, to make sure they start (and stay) the right distance apart.
now if there's nocorners involved, you can make the carriage and train both into a single track train.
ok i hope that helps