Having done model RR for a lot longer than I have done HA, I think I can point you in the right direction. There are several great books out there and some deal with diorama making as well as model RR terrain. The key to good, light weight terrain is the understructure. Blue or pink foam insulation is the shiznitz when it comes to basing large areas. And it's easy to form. Plaster can be kept to a minimum. Plaster land forms are going to be somewhat heavy no matter what you do - and that's why most model RR's build up their landforms with formers and cardboard strip. That way a thin layer of plaster cloth/bandage is all that's necessary to create your hills and valleys. CeluClay - which is a papermache type of material is also very popular for a landscape base material - without the weight of plaster. Oh yes, the books: 1) "How to Build Realistic Model RR Scenery" by Dave Frary, Model Railroader Press; 2) "Terrain Modeling" by Richard Windrow, Osprey Press; 3) "Advanced Terrain Modeling"  by Richard Windrow, Osprey Press. All are available at Amazon and are reasonably priced. These are the standard reference works used by most advanced terrain modelers that I am familiar with. Very good resources for whatever kind of terrain you wish to do.

Eagles may soar, but weasels don't get sucked into jet engines.
molds I have : 40,43,44,52,53,54,55,56,60,61,63,64,65,70,71,72,73,
74,75,76,77,78,85,100,164,202,230,235,250,268,701.
http://miniaturerealities.weebly.com