1/2" plywood is sufficiently strong to hold N scale trains, the problem is that it will tend to bend and warp without supports (piers) every 12" or so. But the alcove span is 3'9" and I didn't want to build a support grid underneath because I wanted to permit maintenance access to the helix that will later be built in the alcove.
The solution is to use steel L-girders to force the plywood to stay stiff and level. Two L-girders are attached together using nut/bolt/washers and short (1/2") screws with a very wide head are used to attach the L-girders to the plywood. The plywood is supported at both ends by wooden piers. Once constructed test that it is level and adjust as needed (thin plastic shims may prove useful here).
This particular segment of plywood subroadbed was more interesting than usual because on the left side you can (sort of) see that the subroadbed divides into three parts, each with its own grade. I've mentioned this design element briefly in previous posts, but after this section is complete I'll add better photos and describe it in detail.
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