A couple of early thoughts on polyurethane foam as a modelling material: first, it's not easy to stick; second, it stinks!
On the plus side you can build hills and embankments with fabulous ease. Sod the scrunched-up newspaper and chicken wire approach! Hey, I know that must be really old hat but two 8x4 sheets, one of 100mm and one of 25mm, look like doing the bulk of the really tricky parts of my layout - including a quarry working - and I am soooooo happy!
Guy