It is the time of year when I can set up my layout again. The Sub-Structure is very rigid. The 1x4 braces are held in place by the way they rest against the wall and dig into the carpet. Friction and gravity do the rest. These braces are bolted to the joists with a single nut. The joists are pinned to the long cleat with big finish nails and glue, which is then screwed to the wall studs. When the panels are screwed on top. Everything squares up, and is very sturdy.
My Helpers then put the Layout itself on the Sub-Structure, and fire it up. I plan to add more track by modeling up. Mountains, bridges and tunnels are planned for the reconfiguration, as well as a loop, so I can just watch it go in circles if I want. I'll keep up with regular updates on this summer project.