After building the East-West cross-city line connecting western suburban lines, which until then terminated at "Charlottenburg" station with eastern suburban lines which had terminated at "Frankfurter Bahnhof" (later "Schlesischer Bahnhof"), the logical next step was a North-South cross-city line connecting the northern suburban lines terminating at "Stettiner Bahnhof" with the southern suburban lines terminating at the subsidiary stations of the Berlin Potsdamer Bahnhof.