The Cologne Stadtbahn is a light rail system in the German city of Cologne, including several surrounding cities of the Cologne Bonn Region (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.