Flymo said "I certainly hope that the pink extruded polystyrene is suitable for a P4 layout as it's exactly what I'm planning on using".
That makes me feel a lot better, knowing I don't have to dismiss the material out of hand. I feel confident in using pink foam. In the past though my "baseboards" were just slices of foam cut to shape and any land contours that were needed were just extra pieces of pink foam cut to shape and stuck on top of each other. Crude but effective if not without problems...
I really think I need to put a bit more thought into this one this time. Somewhere at home I have an article in an old N scale magazine that described working in pink foam in great detail. In that 1" thick foam was used to produce a box girder type structure. I'll have to look into that. Not only will be using one inch thick foam be easier than using two inch. It will also be cheaper.
Once I have the baseboard complete I will then lay some cork sheet on the surface because in my experience pink foam baseboards can be rather noisy.