My 10 by 20 foot modular layout is 2inch foam on top of 2 x 4 framing. It has been in use for 16 years and been through half a dozen major moves with minimal damage. I love the light weight and being foam it makes drilling and adding scenary a breeze. I have cork roadbed and used a combination of glue and track nails with no problems. The only problem I have had is the foam separating in spots from the plywood frame, but some glue and clamps fixed that.
Miketg