I read somewhere (can't remember where) that at one time the full-deck turntable was NOT all that uncommon. Some apparently to keep the snow out (as Glen said) but also some, probably in busy engine terminals, were to keep people from falling in. I don't know whether a full deck with a gallows on top is prototypical or not --- I do think I remember seeing a photo of one --- but since the gallows was one of several ways to support the turntable, I imagine that there could have been some, particularly in the 19th century.
The Atlas TT's mechanism is so foolproof and inexpensive that I think it is well worth using as a basis for constructing a TT of any sort, whether a full-deck or a pit type (a year or so ago, either MRR or RMC had an article showing how to make a pit TT from an Atlas.) The old Atlas TT only had stops every 30 degrees, and that wasn't all that great, but the new ones have 15 degree positions, and that's not bad at all.
BillS