Hi guys,
I have this project and I lack experience in things (when you look at photos you’ll know I lack CAD experience for sure 
).
First: professional overall situation draw:
Second: Draft of the car→gt2 attachment idea:
All not to scale above.
So, the idea is, that I will tension GT2 belt in the shape as seen on the first draw. On the wall, attached in those three places. Those places are also stops. Belt teeth down.
On the second photo, the round thing is bearing, which, I think should be somehow tensioned (rubber, maybe print, spring?) and it’s job will be to push belt down into the pulley (not on the draw yet), and between them belt will go.
Of course pulley will be attached to small DC engine (or stepper if weight allows). On the CAD I got bolt up, but I’ve seen little engines with bolts to the side, which would probably make things easier. Ah, and I will obviously check grub screws on the pulley before testing
Then below of this mechanism will be some kind of box for letters or small toys they will be able to send to each other. Batteries should be build in in that box somewhere and there should be (that’s currently the idea) button which will send car on the way. One push = nearest station, two pushes, end station. After pushing button it will wait for few seconds for second push, but if there will be none, it will just start.
Depending what I will use as engine stations will have endstops or will be programmed in soft as number of steps.
Please let me know your thoughts and ideas! 