Train carriage with 2 bogies following a path

[arcadeperfect]

Isn't difference of Absolute positions of two objects on spline equal to distance between those two?
Or use Relative position combined with spline length?

The problem is that in this case you need to calculate the arc length from the chord length in order to know where on the spline in terms of TF the bogeys will fall, which isn't a simple thing to calculate for a spline.