U1 is a line on the Berlin U-Bahn, which is 8.8 kilometres (5.5 mi) long and has 13 stations. Its traditional line designation was BII. It runs east–west and its eastern terminus is Warschauer Straße S-Bahn station where it connects to the Schlesische Bahn. From there it runs through Kreuzberg via Gleisdreieck and Wittenbergplatz on to the Kurfürstendamm.

Gleisdreieck is an elevated Berlin U-Bahn station located on a viaduct in the Kreuzberg district, and served by lines U1, U2, and U3. The U1/U3 platform is at a higher level than, and perpendicular to, that of the U2.