We have a working prototype!! The cars each have separate cables on their own drums (one under wind and one over wind), on a single motor and axle, with a toggle switch activated as each car reaches the top to reverse the direction of the motor.
We got this working last Saturday at Josh’s and ran it for about 20 minutes without any hick-ups!! A little rough around the edges, and still lots of work to do and details to work out, but at least the basic mechanism works!!