Rick,
Do a Google image search for "Lionel display layout". You will find tons of examples.
IMO the key features of a classic toy train mountain are:
1. A mountain/tunnel situated on a rear corner of the layout.
2. Not much detail - simply plaster gauze material or plaster covered paper towels draped over a basic form.
3. Painted either solid green or some green/brown/tan pattern - again little detail.
4. Very limited use of any ground cover - maybe some lichen clumps.
While there were, no doubt, more realistic examples in the postwar era, notably the 1949 showroom layout in Manhattan, most were very simple.
Jim