U55 was an U-Bahn line in the German capital city of Berlin. It connected the new Berlin Hauptbahnhof, or main railway station, to an interchange with the S-Bahn at Brandenburger Tor. It had only three stations, did not connect to any other U-Bahn line, and was operated as a shuttle line using a single train.

Berlin Hauptbahnhof is the main railway station in Berlin, Germany. It came into full operation two days after a ceremonial opening on 26 May 2006. It is located on the site of the historic Lehrter Bahnhof, and on the Berlin S-Bahn suburban railway. The station is owned by DB InfraGO, a subsidiary of Deutsche Bahn AG, and is classified as a Category 1 station, one of 21 in Germany and four in Berlin, the others being Berlin Gesundbrunnen, Berlin Südkreuz and Berlin Ostbahnhof.