the Aussenalster, an artificial lake. The two outer platforms remain unused since their construction and were originally intended for a proposed U-Bahn line between Altona/Lurup and Winterhude/City Nord (see above). Operating company The system is owned and operated by the Hamburger Hochbahn (HHA), except the Garstedt – Norderstedt Mitte section, which is owned by the Verkehrsgesellschaft Norderstedt (VGN) but also operated by the Hamburger Hochbahn. Rolling stock The Hamburg U-Bahn uses standard gauge electric multiple units that run on third rail with 750 volts DC. The current fleet mostly consists of trains that had been developed from the 1950s to 1980s. In 2012, newly developed trains were introduced. A typical Hamburg U-Bahn train is made up of six (U3 line), eight or nine cars (all other lines). Trains with four cars can also be found during weekends or late nights. Current rolling stock Type DT3 The Type DT3 was introduced in 1968 to shorten the journey time on the U1 line. A total of 127 units were built from 1966 to 1971. 68 units were rebuilt from 1994 to 2001 and received a new front design, they are called DT3E (E = ertüchtigt = rebuilt, in this case). Six units (921–926) kept their original red front designed in the 1960s, they were equipped with LZB for semi-automatic operation between 1981 and 1985. The other 59 units were retired and scrapped due to structural issues and were replaced by new DT4 trains between 1995 and 2003, one unit (909) is being preserved to be restored and used as a museum train once all DT3 are retired. Between 2008 and 2011, most of the 68 remaining units received an interior refurbishing once again. The prototype unit and the last built unit were converted to induction motors around 1980 (being called DT3-DAT from then), in order to test this new technology for the upcoming order of the new DT4 series. They operated in revenue service until 1989 and were scrapped in 1995 (932) and 2000 (931). As the DT3 is currently the oldest train type in the system, new DT5 trains are replacing them since 2013 and retirement has started in late 2015. By July 2018 28 of once 127 DT3 trains are still in service, only running on additional rush-hour services as regular service ended in Summer 2017. All six DT3-LZB units were retired in December 2016 because of their rusting bodies