I've got a fact! In the early 1950s, Germany was already divided into East and West Germany, and Berlin was, too, but it was still possible to travel between the two countries. East and West Berlin each had their own tram company (BVG East and BVG West), and all lines that crossed the border were shared by the two companies. If a BVG West tram made its way to East Berlin, it had to adhere to the local laws there, obviously, and BVG East trams had to adhere to West Berlin laws when they were there. This caused an issue in 1953. In East Berlin, women were allowed to drive trams, but in West Berlin, they weren't. West Berlin started rejecting BVG East trams driven by women, and all passengers had to change to BVG West trams at the border. This is one of the reasons why, beginning in 1953, East and West Berlin both ended their tram lines at the border.