Quite a big update from the last post: I've added a layer of patching compound to the hills, and primed the baseboard. The plaster layer went on fairly easily, I just massaged it into the cloth. I did have to cover the track with masking tape before I started though.
Note to future self: prime the boards before cutting them, and then do touch-ups later!
Next comes some small patches around the bases of bridges, and putting a coat of brown base paint (which as you can see I've tested on the small hill) over everything.