Let me put my 2 cents in here. I am in the process of trying to model a sort-of NYCTA subway, with eventual street transportation (buses), and an elevated line; in other words, a triple-decker.
Right now I'm working on the subway part, in HO scale. My two biggest problems are that 1) nobody is making the car bodies I need (R1/9) in HO at the moment; and 2) just about everything I find that might be useful from the Walther's catalog isn't in stock, and won't be in this century. Also, being located in Arizona is a bit if a handicap.
On the bright side, I have found what I consider to be the ideal pre-fab tunnel material. It is a plastic wire conduit I found in the Grainger (tool co.) catalog, comes in several sizes, and looks like subway tunnels. Needs paint. Grainger #5A679 for the 2" x 2".
Looking forward to further communiques on this subject.
Jeff Trevas