The S-Bahn Hamburg is a suburban railway system operated by Deutsche Bahn. It uses direct current (1,200 Volts) supplied by a third rail, therefore the S-Bahn Hamburg is a closed system. As a result of this, the S-Bahn Hamburg has always required its own vehicle types since the DC system was inaugurated in 1941. At the end of 2007, the first dual-system trains were put into service, which enable the connection to the "normal" DB network with overhead wires.