U6 is a Berlin U-Bahn line, 19.9 km (12.4 mi) long line with 29 stations. It runs in a north-south direction from the Berlin locality of Tegel in the north via Friedrichstraße to Mariendorf, a locality in the southern part of the city. It is a so-called large profile ("Großprofil") line. During the Cold War, both U6 termini were in the former West Berlin but the line passed under East Berlin for a short section of its route. Five of its stations were sealed off by East German authorities and the trains went through these so-called “ghost stations” without stopping, while a sixth, Friedrichstraße, remained open primarily as a transfer station between the U6 and the S-Bahn lines using the north-south S-Bahn tunnel, but also as an official border crossing between East and West Berlin. It was formerly named "CI" from 1923 to 1928.
