I have seen this somewhere in the wild, perhaps a space-efficient storage system in lieu of two shelving units. I'm trying to reproduce, and failing.
Imagine two stacks of trays which hold things that must not fall out, so the trays must remain horizontal. The trays are all connected to the same track, which loops around, so that the first stack is on "the way down" for the track, and the second stack on "the way up".
Q: how are the trays connected to the track, so that they can move along the track eternally without ever tipping over?
I sort of remember that there is a wheel on each tray that counteracts the rotation coming from the track, but I don't remember how.