Train carriage with 2 bogies following a path

[velikizlivuk]

Once both are connected just move first one while second Absolut position if first one plus some distance? If moving along spline direction +distance if against then -distance or vice versa not sure about that. You can make distance random in range or depending on speed to achieve more realistic train like movement. Do this in Update or FixedUpdate depending on movement type (transform or rigidbody).