For version 3 i made a trike-dolly for beeing driven on two pipe-tracks.
I built it from parts of the other dollies, so it consits of the following parts:
3x M8 bolts
3x M8 nuts
3x M8 shims
9x distance pipes from skateboard wheels (those that are inside the wheel)
6x skateboard bearings
As wagon i used a 20x20mm squared aluminium pipe
If you want it to be drivable via a motor, you can use a gearboxed motor or a modded servo like i did
And obviously, two pipes for beeing used as tracks.
You can also use different parts to set the distance between the bearings, but if you got small pipes, you can set the distance in steps, from none to a shim(good for using it as a cablecam) .
Or you can put one pipe between for medium thick pipes up to 3cm/1.5inches to three distance pipes for big diameters..
By doing that you can make sure that the wagon centers itself, otherwise it could jump out of the track.
I think the final version will probably made of two stripboard PCBs and will inlude servo-mounts, axle-mounts, an ATtiny85.
Probably dipped in resin for waterprotection.