String monorail car thing

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.