A material that I became acquainted with through my Dad's use of it for creating indoor signage, is "Sintra", which is 1/8" thick, black plastic sheet material. I get it in large sheets from Tap Plastics, and then cut out the area to be used to fit on top of my styrofoam structures.
You can see the black plastic glued to the top of my styrofoam areas:
Obviously, having a rigid foundation is nice to ensure a nice, level and firm sub-roadbed. The disadvantage of this material is that it does seem to conduct track noise a bit more than other materials, but its something I can live with.
I also use this material to construct some of the custom bridges/overpasses shown below.