I use a mix of 1 inch and 2 inch foam over L-girder benchwork on my layout. The 2-inch foam was available in 2 ft x 8 ft pieces which I could fit into my Ford Escort wagon, but the 1-inch foam only came in 4 ft x 8 ft sheets. If it had been plywood, Ii would have tied it to the top of the car, but the thin foam would have shredded pretty quick. I had brought along a utility knife, so I laid the foam down in the parking lot at Lowe's and cut it in half. I found that the foam was thoughtfully pre-scored at the factory at 24 inches on one side and 16 and 32 inches on the other, so it was quick & easy.
Most of my layout is on 2-inch foam, but I used 2 layers of 1-inch foam in a swampy area where the track is on an embankment above low-lying fields. I marked out the track centerlines, then measured the edges of the right of way from those. Using an old hand saw, I cut nice even embankments-- even around a wye junction. A good planing tool will help with final shaping-- just keep the shop vac handy! I like a foam base instead of plywood because it's easier to include scenic features below track level.
I used Liquid Nails to glue the layers together, and to glue the sandwich to the adjoining 2-inch thick foam. With the messy stuff done, then it was time to start laying track.
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