If I would build a live steamer, I'd use 2.5" scale 7.5" gauge. I have a 4.75" gauge 2-6-2 that has not turned a wheel in years. I do not have the mind set and patience for all the fiddly bits that have to be tinkered on to keep a steamer running. I love to fire friends locos.
However, I am building a 2.5" scale of Goose #7 and have two almost complete 3000 series box cars. I have a 4000 series box under way also.
I have a 1.5" scale model of SV 101 (no tender steps) and use it to pull my 2.5" stuff. It and Goose 7 will be MU-able.
I also have a chassis from a Cli-shay that the original builder did not make to the original Cli-shay plans. The boiler is at the rear of the machine, the engine is in the center and the water tank is all the way to the front (no way to check the water tank without stopping or by using a second water glass and running the lines back from the water tank or some such non-sense). I did have it run some, but he built the engine with no cylinder cocks. After much consideration, I think I'll replace the steam engine with an electric motor and turn it into a box-cab diesel.
I do feel much more comfortable with electronics. Who knows, maybe I'll use an electric motor on my 2-6-2 (the boiler is shot).
Doug