Is 1/2 Inch Rigid Foam Insulation Board Strong Enough Over Open Framing? - Model Railroader Magazine

Frame your lift-out with (nominal) 2x3 steel stud, and put the last part of the 10 footer (bought at Home Depot) across the center.  Put just enough 1/2 inch ply 'cut cookie' in place to support the track and building, then fill the rest of the space with foam.

Believe it or not, the steel is lighter than equivalent wood.  It also comes without checks, splits and knots and simply disregards humidity.  Finally, all your fastener needs can be handled with the tiny screws normally used for steel stud assembly - no long wood screws into end grain.

Tools required?  Tin snips, vice grips and a power drill with crosspoint bit.  Lacking tin snips, you can do your cutting with a hacksaw.

Chuck (Modeling Central Japan in September, 1964 - on steel stud benchwork)