The turntable can snap into 3 positions, in each position it is connecting the opposite track ends.Between the snaps it rotates freely.It has enough room for one regular DUPLO-sized car.

To assemble, just place the rotating part on the turntable's base, and fasten it with the screw.

The screw has a notch so that it won't get loose, or fall out. It also means that you won't be able to easily remove it, so make sure that you are satisfied with the result before making it final :)

For aesthetic reasons there is no drive on the screw - I used two fingers and pushed it hard while fastening it.If needed, I can upload an alternate screw design with drive.

Supports are only necessary for the base, for the connectors.Otherwise there are no quirks, just print it with standard settings (15% infill), and it'll be fine.