With the structure complete it's time to add track to the fiddle yard, and wire them up. The wiring took some thought, given the deck both slides and rotates - the "usual" solution of sliding bolts to conduct power to the aligned track wouldn't work unless there was one for each rail, and that seemed too fiddly. In the end I settled for micro-switches and wiper contacts...
This view of the sliding deck with the rotating deck removed shows the pivot bolt, two micro-switches protruding through at the sides, and six brass pads. The C-shape slot was for passing cables to the deck, but in the end I found that wasn't necessary. The two micro-switches act as a pair to reverse the polarity to the tracks on the deck, a notch in the underside of the rotating deck means one is "on" and one is "off".
Five micro-switches on the sliding deck act to power the aligned track only, which is fed to the rotating deck via the brass pads to the wiper contacts. These are arranged to that as the deck is rotated the tracks are fed from the correct feed - as the rear track becomes the front one, etc. The middle track is split into two "stubs" (e.g. for railcars), so the wipers also ensure only the correct "end" is live. All tracks share a common feed to one rail, which is fed via the pivot bolt - you can see it mounts though a piece of PCB. Strips of PCB across the deck make connecting the wires easy, as well as keeping the tracks in place.
Further PCB strips at the ends of the deck secure the rails firmly. The end of each rail is given a slight chamfer on the inner edge to help in case of slight mis-alignment. The micro-switch rollers give rough alignment as they drop into the notches, but they aren't strong enough to give precise alignment and there is some slack in the construction (!), so a check by putting a finger on the rails is a good idea! There's enough friction that the deck is unlikely to move while a train is moving.
A wide view of the deck, the handles "up" lock the rotating deck to the sliding deck. The stub track on the right end isn't live, but may prevent derailment in the event of an overshoot... though I'll confess both ends don't always align together!
Slid right forward clear of the wall, and the ends of the fiddle yard, and handles "down" form end-stops to the tracks as well as allowing rotation.
There's a fair bit of wire knitting underneath to make this work, and there has been considerable frustration (including a faulty micro-switch, and a couple of misaligned ones) to get it working - but it does actually work now!