This is a tractor that uses these finger treads -> https://www.printables.com/model/1236990-finger-treads.

I first designed the tractor and trailer to be able to separate, but my product manager (i.e. my son) did not approve. He viewed it as "broken". So I have two different models:
1) Tractor and trailer can separate (chuglet_body.stl, chuglet_trailer_body.stl)
2) The tractor and trailer are printed together and are attached where the trailer can still swivel (chuglet_with_trailer_attached.stl)

My product manager was right, the second option is my preferred version.