Alan:
Thank you for your kind reply.
In short, there will be a lot of steel. More specifics:
My attic is a huge unobstructed space with a peak 16 ft above the third-story attic floor. The train room will be built as an insulated room-within-a-room approximately 25x35 ft, with 8ft walls, at least 5 1/2" thick. I will have to use an exterior crane or lift to bring in material (especially sheet rock, plywood, and engineered ceiling trusses.) It's a bit daunting: I had tried to work up a good solution for a basement (dungeon) layout room, but there were too many expensive obstacles, especially in a 130 year old structure. This option will allow for a dry climate-controlled space with few obstructions. I had thought to lighten the load with steel wall studs (vs. 2x6 wood studs), but the prospect of signal interference makes me leery. I have seen a number of modelers use steel construction for benchwork and for suspended ceilings, but I don't recall any mentions of more extensive use of metal such as I contemplate.
JimW