First to Albertstrains who posted earlier in this thread - What a Cool Mountain!
I decided on using this 2" Styrofoam blue board from Lowe's. $22 for a 4' x 8' sheet.
It's light, cuts fairly easy and cleanly. Looking forward to sculpting some landscapes!
Now to revise my track plan for the placement of accessories and buildings purchased over the years.
Planning on three major focal points. Lionel Lighthouse. Lionel CC Gantry Crane. Atlas Suburban Station (20" long).