The Hanover Stadtbahn is a Stadtbahn (light rail) 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 (Lines 1-11 & 17; along with one night line (Line 10E) and two supplemental lines (Lines 16 & 18)), serving 196 stations (including 19 underground stations, and 119 high-platform 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.