How about looking on rail-sim.de for a start. The Usedom v2.2 is set in 1940s, the original Usedom 1988 is set in era 4 but could be in era 3 as nothing changed in the route. There are other routes and there are plenty of era 3 and era 4 steam and diesels available and rolling stock if you look, for both DR and DB in the DDR and DBR days. The BR86 is era 4 (era 5 was from unification) and is DR NOT as DTG advertise as being DB.
How about Thuringer Bahn Halle Bebra via Fortha and Gerstungen. The current section which is Bebra Gerstugen Eisenach via Herleshausen Wartha is closed due to the route crossing the Inner German Border.
I would be happy, if do you create some German Routes? Did you DTG create at the moment a German ROute, what we can get happy to wait? Its not a problem if they have some weaks.The community have the power to solve it. We missed a creator who handles the train with trowel and whistle.