Majority of the the time I see trains run into a dark empty tunnel and thought it would be nice to build a tunnel with a lighted and finished interior. I mean, let's give the folks on the train something to look at (hehe!). So here is my impression such tunnel:
This video shows how mountains were made to fit over tunnels using Great Stuff:
You can see a gap where the portal and mountain join. I have some green bush material that fills that gap but omitted it here for viewing.