The Cologne Stadtbahn is a light rail system in the German city of Cologne, including several surrounding cities (Bergisch-Gladbach, Bonn, Bornheim, Brühl, Frechen, Hürth, Leverkusen-Schlebusch, Wesseling). The term Stadtbahn denotes a system that encompasses elements of trams as well as an underground railway network (U-Bahn) and interurban rail, even including three lines that are licensed as heavy rail and used by freight trains as well as Stadtbahn vehicles. Two of these lines connect the Cologne Stadtbahn to the Bonn Stadtbahn. These lines (16 and 18) are jointly operated by both cities' transport authorities, resulting in both systems and the lines connecting them sometimes collectively referred to as Stadtbahn Rhein-Sieg. The Cologne Stadtbahn is operated by the Kölner Verkehrsbetriebe (KVB) and the Bonn Stadtbahn is operated by the Stadtwerke Bonn (SWB - City of Bonn Utilities Division). The KVB and SWB are members of the Verkehrsverbund Rhein-Sieg (VRS - Rhein-Sieg Transit Authority), formed in 1987 to consolidate the transit authorities in the metropolitan Cologne area and operate a joint fare structure. There are 38 underground stations, 4 more are currently under construction.