Its plentiful, it's cheap and we've all got plenty of it (No shouting at the back, you who buys bagged cereal I'm watching you lol)
I've knocked together a pretty serviceable platform out of one of the narrow edges. As well as some kind of halt to try out the tarmac surfacing. Check them out here!
1. First is the platform. this would be good as a small intermediate stop on a branch somewhere. Doesn't even need a building, or you could say the building is up a path off scene. Just a small shelter will suffice, really.
2. And the bracing pieces underneath. I can't really understate how important it it to brace this type of cardboard, as it's quite thin and bends in ways you really don't expect. One idea is to laminate it, double strips and lay it down under a heavy book or something. Something big and flat anyway.
3. And the halt, made out of a few offcuts of cardboard. Not the tidiest but it'd be ok as a halt on a diorama or something. As long as you don't look too closely...
4. And the underside