A while back I made a string monorail car type thing with an Erector set and a piece of wood. I happen to have found it in one of my old storage boxes. It didn't run reliably on a sloped line and was too heavy causing the line to snap if i tightened it enough to keep it from bending downward (darn gravity).
So I decided to pop open the Sketchup and design a replacement.
There are two files: the "Monorail car thing MK2.stl" and the "Monorail car thing base.stl".
Print two of the Monorail car thing MK2.stl and optionally one of the Monorail car thing base.stl.
Attach the two Monorail car thing MK2.stl as shown in the picture, assemble the wheels as pictured, and then you can either attach a motor and battery as I did or have a manually operated car with the Monorail car thing base.stl, which has a hook to attach a cup or similar item.
Unfortunately I cannot find any source files. If you need it I can probably try to dig it up or remake it, it's not that complicated and I remember the dimensions.