To elaborate on the "political geography", the U7 and the U9 were the only lines of substantial length that for the most part opened after the division of Berlin and are centered on West Berlin's center around Charlottenburg, rather than the prewar/modern center of Mitte.

Here's an interesting BVG map from 1977, showing the divided network and U7 under construction: https://transitmap.net/west-berlin-1977/ Note that pretty much every major line other than U7 and U9 has an abandoned or pass-through segment due to the division.