I have used nothing BUT foam on my layout.
I have my layout built on a solid wood table framed with 2x4’s and covered with 1/2” plywood and then 2” white foam board(cheaper than pink or blue foamboard).
All my grades and upper levels are made from this material with some 3” as well.
Check out my CPR Mountain division thread and you can see how I approached building my layout
Thanks Albert. Very interesting I will check it out. Thanks,