A sort of spiritual counterpart (capitalist/communist, yin/yang, Luke/Vader, take your pick) to U7 is U5, the only line built by East Germany: https://en.wikipedia.org/wiki/U5_(Berlin_U-Bahn) Colored what a friend of mine calls scheisse brown, it sprouts out of central East Berlin (Alexanderplatz) far out into the countryside, with forests, fields and dense clusters of commieblocks tightly packed around stations.
And these days it goes all the way to Hauptbahnhof!
And Turmstraße and Jungfernheide are already prepared for crossing it.