Marvelous as insulating foam is, it still needs a surface coat of plaster to smooth things out.
I picked some Homebase own brand filler. Into this I squirted some brown emulsion from a match pot, then made the filler up with water as per the instructions. A stiff mix works better than a sloppy one.
Application is just like covering a cake with icing, which might not help explain but go with it. Take a nice flexible plastic thing - one of those fake credit card things that appear in junk mail is good for this (mine came from LoveFilm). Pick up a dollop of filler and slap it on the foam. Then smooth it around. A tile adhesive comb is handy to have around for sharpening up edges and the little glue spreader gets into the nooks and cranies around the bridge.
Clever modellers will protect the track work with some masking tape before starting. I didn't but only got a couple of blobs in the rails and brushing them immediately with water washed the stuff away. If you don't, clipping it off won't be nearly as easy.
The filler takes 24 hours to properly harden and goes a lighter brown when it does so you can see the state of play. I love this bit of layout building, the model takes shape before your eyes and it's proper messy.