I try to keep it light, but use a sandwich of materials. Immediately on top of bench work I used a 3/4” maple/birch plywood with a foam core. This material is super light, and very flat, but you can still screw into it well. On top of this I glue 1-2” of styrofoam - the pink or blue or green stuff. Sometimes I use two layers of 1” for various terrain or just a single sheet of 1” or 2”. Finally, I install 1/2” Homasote with glue. The Homasote is roughly the size of the roadbed and I route a channel into the foam to recess it. The depth of the recess varies from 1/2” on industrial sidings where track would be flush to less for branch and main line track, where roadbed would be raised. I am working with 2 rail handlaid track, so if you were doing 3 rail, you could just stop with foam. I’m going to try to attach a photo of the materials I’m presently cutting for a shelf layout - From left to right - 1/2 Homasote; 1” foam; 3/4” maple plywood with foam core.
