I believe that The Talyllyn would be nice, as it would get Wales, Narrow Gauge and Steam out of the way, it's a short line, and it has historic significance, because it was the first preserved line. With it being run by mostly steam engines, it could be a good introduction to multiplayer, where one person is the driver with the other a fireman.
Bit too complex for a "starter" multiplayer. Either way, would be a nice addition I suppose, something different.
I agree with this suggestion, I've ridden on the line many times and it's very scenic with some interesting stops!
