There's just no easy way to conceal an access panel on the top of a layout. You have to disguise a rectangular panel with a natural-looking landscape, and I just couldn't figure out out to do it convincingly. So out comes the drill and the saw, and there's a hole behind the tunnel and in the side wall for access. I put plaster cloth over it all, and I think it looks much better than trying to have access from the top.
I didn't leave much room to the side of the curve that runs alongside this hill; I'll have to be very careful when surfacing it not to run out of side clearance.
Here's a shot of the entire layout. All the hills are covered in plaster cloth, next comes spreading a thin layer of material to seal it, fill in gaps here and there, and give it the right texture.