Thanks for the good advice! I read some posts about SGMA's module specs and was thinking about two standard gauge loops using Std72 and Std42 curves. The idea behind these two curve sizes would be to provide adequate spacing while keeping the tracks somewhat close to the room walls. I guess that Std57 is too small for a parallel track using Std42 curves. I hear universally good things about Ross switches, but they are a bit expensive. Do the MTH tinplate switches work with most modern tinplate, or are there derailment problems? I am strongly considering flexxbed instead of homasote.
Then there is the question about O gauge track for tinplate - thinking about gargraves and/or tinplate. The O gauge track might run slightly above and behind the standard gauge track, but closer to the walls of the room.
Once the carpet is installed, I will begin some sort of shelf/table. I may just have to start and see where it goes.