Drag object along the VA path

Hi,
I’d like to have some graphic object which is possible to drag along the VA path.
I’ve tried something to do but no success. The path is a random up-down hills.
Exactly like the Motion guide does it.

Maybe some solution: put some object on the motion guide, create some possition
sniffer, apply position to the drag-object… but there’s the problem how to obtain
the direction of the drag… even when I use Beziere curve, the right direction still persists…

Any idea ? thx