Cardboard Mockup
So I started on some cardboard mock-ups of my lower level subroadbed. I chose to start over in the corner where I wanted to place a turntable scene. I imagine that this will be a full sheet of 3/4 plywood set into that corner of the shed, and have its one inner corner shaved down to meet the narrowing portion the freight yard scene running down that side of the room.
I cut the cardboard about an 1.5 inches bigger in radius than the 24 inch radius of track (51" diameter) that I expect to place in this region (to give clearance from the backdrops that will be on the walls of the shed, and to give some clearance at the aisle edges.
(I wanted to maximize the shapes so I could tell if they would be to restrictive to moving around in the overall track plan,...can always make things smaller, but likely no larger).
I supported this big piece with some very stout steel brackets mounted to the 2x4 studs of the wall (note: backdrop sheets of masonite are not installed yet). I chose these brackets for there large size (16x18), and the fact that they have a 'open area' that will likely be utilized to further support my staging tracks just below that overhead subroadbed of plywood.
There is a really nice big open area under this cantilevered plywood subroadbed. It is also a very 'deep' lower level shelf to try and reach over to work on any backdrop, and/or upper level scenery. So while I feel the 3/4 inch plywood is strong enough to support the trains, scenery, structures themselves, it would not stand up to any climbing upon by myself. Then I thought, why not just make up removable, repositional, supports for the outer edges of this big deep shelf ( I represent just one such support with that cardboard upright in the photo. I imagine the real ones might just be 2x4 constructions, or perhaps nice big round PVC tubes.
I am also considering placing a long rectangular flat strips of 3/4" plywood between those metal brackets and the subroadbed sheet. This would lend additional support to the cantilevered subroadbed, as well as provide some clearance for the DCC bus wires attached to the bottom of the subroadbed.