Hi,
As to locations for gallows turntable, I believe that Mashall pass had one. I also believe that the Silverton RR had one at Corkscrew Gulch.
I would suspect that Durango's original one was a 50' gallows.
I also believe that the Gainesville Jefferson & Southern had a 50' gallows in Gainesville, GA and maybe one in Monroe, GA.
An "A" frame turntable is at laws on the C&C and I think the Maine 2'ers use a smaller "A" frame design (more like a king post truss bridge".
Doug vV
PS
I also suspect that Earl is right that the gallows came in 50' length only. The kit I purchased circa 1970 was 9" on length in HO and was probably 9" to fit more locomotives than to be 100% historically accurate.
It's superstructure could be placed on a 9" Atlas turntable for an earlier period and powered or unpowered service.
A 50' gallows works out to be about 6.65".