I think the West Anglia Mainline is quite a well-suited route for Train Sim World 3. There are short-distance and long-distance routes, but not unreasonably long. The line has seen plenty variety of traction over the years and I personally don't mind what traction would be added should the route be published. Though again, I think for TSW 3 the route itself would fit in quite nicely.
That means King's Cross Hitchin Cambridge then King's Lynn. One advantage of running direct from Cambridge to Liverpool street is that you can actually operate Stansted Airport Express
That's the east coast mainline, that's a completely different line to what I was suggesting. It could work though, especially the non stop trains to Cambridge.
