Regardig routes, the good news is that there Will be a lot of time for someone to make one before I finish the engine
French routes would be interesting in a slightly pre-ww1 timeframe. But that is a personal opinion - I suspect the 30's will be more populair with others. Concerning the selection of a line, these engines, in small numbers, saw service with the Nord, Midi, Ouest and Est. A slightly enlarged type was used by the Paris-Orleans. Only the PLM had their own design.
If you want the uk ones, the riviera line is a good place to start I'd think. My prime reservation is that there's nearly no technical information on what equipment was added and removed to GWR-ify these locomotives. But then I havent really asked anyone.
Finally it should be known that unlike others I dont have commercial obligations to make sure there's a route for the thing to be run on.