Make locomotive Pull Carriage

Hi:

I am currently at the begginnings of building a train simulator in Unity, Currently the locomotive is a simple rigid-body on wheel colliders and the coach is the same(but motor-less). i Want the locomotive to be able pull and push the coach.

now there should always be some kind of slack between the loco and coach(the loco has to move a few inches before the coach starts) so i think that a spring joint with one anchor on the loco and another on the car with Super Strong spring and dampener should do the work but whatever i do, the car doesn't move, how can i move it?