Automating a turntable is a challenge, because of the way the Fleischmann turntable (essentially the only commercial model available for N-gauge) works. It's driven by a DC motor using a kind of latch pin released by a solenoid and detected by a microswitch to stop it exactly in the right places. The drive system only knows the turntable is in a legal parking position but not which one. There are mods you can do by fitting a stepper motor or by replacing the mechanical latch with an optical encoder disc, but either way it's a lot of messing about. 

After trying to lash something up with diodes, relays and reed switches, I tried the Littfinski TT-DEC, a decoder that takes the relatively crude (and noisy) approach of letting the latch pin fall into each notch as it passes, thus reliably tracking how many positions it has moved. Whilst the motion isn't silky smooth, it isn't as jerky as you might expect, and the only real disadvantage is the constant clicking of the pin as the table moves.