The ring line with a length of 20.68 kilometres (12.85 mi) serves 25 stations.

[1][2][3] The ring line was built from 1906 to 1912 and had a length of 17.48 km (10.86 mi) and with 23 stations.

On 2 January 1967 the first part of the line to Billstedt opened with the section leading to Horner Rennbahn.

[3] In 2009 eastern parts of U2 and U3 lines were swapped behind Berliner Tor.

Since then, it ends in Mümmelmannsberg, and the U3 became a ring line again with a branch to Wandsbek-Gartenstadt.