U6 (Berlin U-Bahn)

U6 is a 19.9 km (12.4 mi) long rapid transit line on the Berlin U-Bahn 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 one of the five large profile ("Großprofil") lines.

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 "C^I" from 1923 to 1928.