The train I've wanted most in train sim world since the first steam train released, and i'm sure i'm not the only one, is a4 4468 Mallard. I think it would be an incredible train to have in this game, it's definitely one of the most iconic steam trains and would be a great addition and nice to drive. As for the route for it to be included in, i think it should be based on the special service it did from doncaster to clapham junction. Heck even if it was just one of the a4 engines in general it would be great. But i think mallard would just be the most iconic one to choose. While i know They haven't been big on doing steam trains recently, However i think they just can't ignore an opportunity for what would be an incredible addition to the game for us brits that enjoy steam trains.
