The West Coast Mainline would be a good route. You have big stations such as London Euston, Crewe, Preston etc plus you got Carnforth which is also main base for West Coast Railway Company so you got steam and west coast diesels. You currently get Virgin Trains 390s pendalino, 221 voyagers, 4x Caledonia Sleepers that does the whole distance plus a large amount of Frieghts, auto train, MoD, Royal Mail EMUs and flask/nuclear waste. Also Crewe you get transport for Wales, Cross Country, Northern, East Midland Railway and London Northwestern Railway with near by Drs depots Preston you get Northern, Virgin Trains and Transpennie Express Glasgow Central you got ScotRail, Cross Country, Transpennine Express, Virgin Trains, London North Eastern and Caledonia Sleepers. Personally I would like to see a map that is from London Euston to Preston calling at Crewe.
I would love that too, but unfortunately, this is not possible at this moment. Entire WCML (or ECML for that matter) would take years to develop and the cost would be your regional equivalent of ~400 Euros. I really hope we get Glasgow-Carlisle or London-Birmingham one day.
I wasn't on about the full Mainline itself as a map was just trying to point some of the things that Run along the WCML and some of big stations that could make a good map.
Dude do you listen to anyone TSW DONT MAKE LONG ROUTES CAUSE IT WILL TAKE LONG AND THE PRICE WILL BE EXPENSIVE it would be like £100 to £150