Turntables obviously need to reverse polarity of their rails. In the bad old days, people used to use all sorts of tricks to switch the polarity. One could use a split ring rail and opposing wipers, or a manual switch with DCC, but in either case, the engine sound will cut out briefly. It’s much better to use one of the auto-reversers.
Thanks to Brian Pate, who is sadly tearing out his fabulous Klondike Mines Railway and Canadian Pacific layouts, I lucked into an OnGuard DCC Circuit Auto Reverser from DCC Specialties. It was trivial to wire it to the wipers that deliver power to the turntable.
I then gathererd all the feeders from the roundhouse tracks and the lead, and buried them in a trench in the underside of the baseboard. This trench enables the roundhouse and turntable baseboard to sit on the layer of plywood below. The feeders all terminate in a terminal strip that is in a location where there is only one layer of plywood. The idea is that I should be able to remove the whole thing to work on the roundhouse with minimal rewiring effort.
While I was in a digging mood, I also tested the baseboard to ensure the turntable locking mechanism fits through the hole. It does, and I also drilled a hole through the benchwork for the choke cable that will drive it. It unfortunately looks like I will have to terminate the lock off the center-line of the fascia, unless I want a bell crank.