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. The first ideas for this project emerged only 10 years after the completion of the East-West cross-city line, with several concrete proposals resulting from a 1909 competition held by the Berlin city administration. Another concrete proposal, already very close to the final realisation, was put forward in 1926 by Professor Jenicke of Breslau university.