Personally I like foam. I try to use the thickest sheets I can for the project. Mountains I use 2" thick. For flatter areas like you are doing the 1" works best. I shape my foam with Hot Wire Foam Factory carving tools, Fusion Fiber (would work really good for your bridges scene), latex interior paint and the texture from Scenic Express. I do most of my carving, painting and initial texture outside, then install and finish.
I really don't like the mess of plastering, painting or gluing. I use spray contact glue to stack my pieces of foam together. To help keep the foam sheets from separating, only use very flat pieces and cut off the edges that are warped from the manufacturing processing, Glue will not hold two warped pieces of foam together. Construction glue can not be carved or cut with hot wire tools, white glue will never set up between sheets of foam.
