The Hamburg U-Bahn is a rapid transit system serving the cities of Hamburg, Norderstedt and Ahrensburg in Germany. Although technically an underground, most of the system's track length is above ground as with many other U-Bahn networks in Germany. The network is interconnected with the city's S-Bahn system, which also has underground sections. It is operated by Hamburger Hochbahn and belongs to the Hamburger Verkehrsverbund (HVV). It was opened in February 1912 and comprises three lines with a length of 101 km (63 mi) in 2006.