The Hanover Stadtbahn is a Stadtbahn system in the city of Hanover, Lower Saxony, Germany. The Stadtbahn opened on 29 September 1975, gradually replacing the city's tramway network over the course of the following 25 years. Currently, the Hanover Stadtbahn system consists of 12 main lines, serving 201 stations, and operating on 121 kilometres (75 mi) of route. The system is run by üstra, which was originally an abbreviation for Überlandwerke und Straßenbahnen Hannover AG. As of 2007, it transported 125 million passengers per year.