ChicagoCrossingMRR, glad to see you here on TRW. Looking forward to seeing your posts about techniques.
Regarding making tracks disappear, I see prototype tunnels in all sorts of situations that aren't really "mountains". It is really just a matter of whether it is cheaper to make a tunnel than a cut, and maybe a bridge.
So, for instance, there is the B&O mainline tunnel at Point of Rocks where the mainline goes through a tunnel in one direction and, only feet away, around the rock point in the other direction, where the old C&O Canal bed used to be. The rock is stable enough to make a tunnel feasible. There is a tunnel through a hill on the old B&O Georgetown branch at Dalecarlia, with a roadway above. Similarly, there is a tunnel for a branch line in PA where the PA Turnpike goes over the hill and the train goes through the hill.
I even worked in a building, named "The Air Rights Building" where the builder of a multi-story office building won a legal battle to cantilever the building out over the tracks of the B&O Georgetown Branch (which has now become a "rails-to-trails" bike path).
For my own layout planning, where the location is actually mountainous, I am still planning to have one end of the mainline tracks disappear around a bend that follows a river around a hill, rather than going into a tunnel. But, the other end will come out of a tunnel, because staging needs head room, and it is very close.
