The only advice I could venture to offer you would be for you to consider how much space can be consumed by a turntable and roundhouse; also, if you are going to have a yard facility from which the engines would draw freight or passenger consists, that adds a big amount (in my opinion) of switches and lines of track, additionally.
I don't have the impression in my mind that 12'x12' of plywood acreage would afford you space for much more beyond that.
Of course, if that kind of operation is what you have been dreaming about, you will make the whole thing work. Imagination can do a lot.
FrankM
P.S. This acreage started out as such a facility, complete with turntable, roundhouse, and attending RR-yard. Fortunately, I had saved the round cut-out from its center, which was where the turntable went, and plugged it back in for construction of the neighborhood. My wife and I and our guests enjoy this area as it is in this final iteration.
Have fun.
FrankM, Moon Township, USA