Hi all Personally I think they should add class 50s to the game the route they should add to them would be Liverpool-crewe due to them working on the wcml back in the day and I think this would be a very popular loco added to the game.
Apart from the era, Paddington to Reading the most obvious stamping ground as part of a retro pack. To fit the WCML, they would have to be north of Crewe/Preston between 1967 and 1974. In more recent memory, part of the Waterloo to Exeter line would be a decent route to run them on, or possibly South Devon Banks from Exeter or Newton Abbot to Plymouth. The latter has the advantage it could layer with much of the existing classic BR diesel fleet.
Gloucester to Birmingham would be a great route for 50's as every other blue diesel could substitute in. You could also have banking on the Lickey incline.