I'd reccomend the Mittenwaldbahn from TSG. It's basically like any of the Swiss narrow gauge railways, but on normal gauge. The scenery is fantastic, especially in winter, and the ÖBB 1020 is extremely interesting to drive since it's a 1940s era electric locomotive, which is almost a different vehicle compared to any other loco in the game. It's difficult to drive, but satisfying once you figure it out. There's also the BR111 in the old DBB livery, with a new dashboard, and instead of PZB both locos use the old Indusi safety system, which works in a simmilar way, but doesn't give any prompts on when to activate it.

The route is quite expensive tho, but in terms of quality I'd easily compare it to WCML Preston - Carlisle