I just built a 30" turntable in the past month. I needed something large enough to turn my largest locomotive, a MTH Premier Santa Fe Texas. The Atlas turntable is too small at 24" and the available 30-32" units are far too expensive. The bridge is plywood with balsa details. The bridge is screwed to a 4" lazy Susan bearing which is screwed to the turntable pit. The bottom ends of the bridge are 1/8 styrene sheet, which ride on the pit rail (old 027 track). It lets the bridge move freely but holds it in position when it's stopped. Power flows to the turntable via a slip ring connector mounted under the pit. I could have (and still can) motorize it fairly easily but have chosen not to. It's not used every time I run trains so I get away with grabbing the bridge and spinning it by hand.
