It would be great if they put this train in the game if they put in the whole gwe line the cross country trains could come in to service on tsw and then the stansted express 379 could be in tsw it might be confusing but if the whole gwe lime is in the game the cross country train could join up as they do operate at some of the same stations then stansted express operates at stansted as well as cross country so one can come out after another and can connect the UK overhead lines
The GWE map has nothing to do with Stansted Express, you'd have to have the Reading to Birmingham route, then the Birmingham to Leicester Route, then the Leicester to Cambridge Route, then the WAML down to Stansted then finally the WAML to Liverpool Street