The oldest line of the system, the S4, was an extension of an existing line that had been operated by the Karlsruhe Stadtbahn since 1992. It was extended in 1999 from Eppingen to Heilbronn and since 2001 it has continued over the new Heilbronn inner-city line (Heilbronner Innenstadtstrecke). In 2005, the line was extended to Öhringen, which is integrated into the Heilbronner Hohenloher Haller Nahverkehr ("Heilbronn Hohenlohe Hall local transport", HNV), a transport association (Verkehrsverbund) that is responsible for coordinating fares and transport investment. In 2013, the new S42 service was opened to Neckarsulm. Since 2014, this service has continued via Bad Friedrichshall to Sinsheim, while the new S41 service also runs via Bad Friedrichshall to Mosbach.