Managed to find a few images of the P4 one I did. Hopefully you'll be able to follow the sequence but basically the turntable spins on rollers on a copperclad plate which has been gouged to give 4 quadrant sectors, the turntable having segments with plungers to pickup. Only one road is actually useable. The outer rim is a piece of 22mm brown plastic plumbing pipe, and the whole thing screws up from under so it can be quickly dropped out. Power is one of the high-reduction N10 gearmotors fed by rechargeable batteries and is operated by a push button and aligned by eye - I tried a stepper motor but couldn't get any success before it went phut, so simplicity prevailed.
Hope it makes sense.
The assembly - sorry I don't have pics of the cut tracks but easy enough to work out hopefully
Actually a bit earlier, the rim with the deck and the base before gouging the tracking segments
Underside of the deck showing tiny ball rollers mounted on brackets.
A bit out of focus but in position'
Scribed walnut decking added and cobbling beginning
Height adjusted
Drive mechanism, so very slow which allows indexing by eye
Cobbling finished
This was a fun build of a micro P4 layout on an A4 sheet and happily takes 24.5mm SPUD-powered industrial locos. I'll leave you to work out the radius.
Hope this will help the original poster - it's quite easy to build one for any gauge/scale, and you can drive over it.