Two 1mm holes drilled above each LDR allow light to get through from the overhead lighting above the layout. The idea is that when the turntable bridge reaches the 1mm holes, the LDR will go low and the Arduino will know where the tuntable is. From there, it's only a certain number of steps on the stepper to align the tracks. That's the theory, anyway.
Using an LDR at each track. These were mounted under 2 1mm holes, drilled at the center of each track. I didn't want to use a larger hole, as I wanted them to be unnoticeable. However, these didn't trigger in a consistent manner.
Cutting a small section of the turntable pit rail (isolating it), and connecting that to an Arduino input. A wiper on the underside of the turntable bridge would then make contact with this section of the rail and pull it low, which would be detected by the Arduino. This didn't work, as the wipers didn't make sufficient contact, and as such this approach didn't work either. (This also caused the turntable bridge to 'float' off its base.)