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 weekend express 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 of route. The system is run by üstra, which was originally an abbreviation for . As of 2007, it transported 125 million passengers per year. Three types of light rail cars operate on the system, the TW 6000, built from 1974 to 1993, the TW 2000 (the so-called "Silberpfeil"), built from 1997 to 1999, and the TW 3000, which was first introduced into A line service in 2015. The system is extensively used, especially during trade shows on the Hanover fairground like CeBIT or the Hannover Messe. It makes up for more than 60% of the GVH transport association's total traffic, spanning over four cities and two counties. Hanover Stadtbahn is complemented by DB-owned Hanover S-Bahn, a suburban heavy rail network which links the outlying suburbs and towns, as well as Hanover Airport, to the city centre. Network The main Stadtbahn networks The Stadtbahn is a mixture of traditional tramways, of which 82% have been upgraded so far to have their own right-of-way, and an U-Bahn-like system of tunnels in the city centre. It is owned by infra GmbH and covers nearly the whole city area. Parts of the Stadtbahn reach into the neighbouring towns of Garbsen, Isernhagen, Langenhagen, Laatzen and Ronnenberg as well as into the town of Sarstedt, which is in the borough of Hildesheim. The operator's concession is held by üstra Hannoversche Verkehrsbetriebe AG. The system currently consists of three full Stadtbahn route networks, respectively named (with their defining tunnels in brackets): A (Waterlooplatz – Lister Platz) B (Vahrenwald – Döhren) C (Königsworther Platz – Braunschweiger Platz) A fourth tunnel, to be used for the D line (Goethestraße – Sallstraße), has been proposed but has not been realised so far due to the high costs of construction, currently estimated to be around € 1 billion. Nevertheless, the above-ground parts of the D line, most notably the D Süd, connecting the Hanover fairground to the