Newman Atkinson says:

While planning your hills whether you have trains tunneling through or not, Allow access for track maintenance, or wiring maintenance as needed. Rocks can overlap other rocks to hide those cut seams and no one will know. So what ever you plan is great, Just plan it with the access you will need later such as a lift off or side entrance. There is always something you will wish you had done to have that access. I used flip handle drawer pulls to lift mine off and when they are in position they overlap the rocks below so there is no seam to blend in. Once it is in place I have stacked rocks that cover the drawer pulls.
As far as rocks on the side of hills there are a variety of rock molds to form these whether it be the rock cut along the tracks or rocks sticking out of the hillside. Photo backdrops will make that hillside look like it keeps going. City scenes are the same way. As I will have a major city backdrop and buildings on the hillside but will also have tracks underneath. The building section has to be able to lift off for access to those tracks among wiring access for those buildings. Just don’t forget the access for later. You will be glad you did. Newman