C tunnel, have been upgraded to proper Stadtbahn standards, some preparations for interchange with the current stations, such as an empty station below the current Hauptbahnhof station, have also been built in the past. The entire Stadtbahn network uses various colours to differentiate between the main route networks. For example, a station that serves both A and B lines has a blue and a red stripe on its station sign; stop on the D line would feature a lime stripe on the station sign, and so on. This scheme sometimes also recurs in the architecture of the stations, for example Kröpcke station used to feature tiling that changed colour when changing between lines, i.e. yellow elements were introduced in the red tiles when walking from the B to the C part of the station. Individual lines operate within the main route networks, diverting to various terminals at the city's edge. 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 supplementary lines (Lines 16 & 18). The numbers are assigned to the lines as following (lines that only run during exhibitions on the Hanover fairground (i.e. supplementary lines) are denoted in italics): A network – Lines 3, 7, 9 B network – Lines 1, 2, 8, 18 C network – Lines 4, 5, 6, 11, 16 D network – Lines 10, 17 and 10E* (*10E is a weekend express night service line, using parts of the A tunnel) Whilst the number 6 and 16 services use a large part of the D line, they are still denoted as C services by using the colour yellow on station signage. The main hub and transfer point of the network is Kröpcke, a large subterranean station in the city centre. Lines 1–9, 11, 16 and 18 all call at this station, making it possible to reach every point on the A, B or C route networks from every other point with only one change. The D tunnel One proposed major tunnel that was part of the 1965 plans has not been built so far and probably will never be. The city centre tunnel for the D line was originally supposed to run from a ramp at Goethestraße via Steintor – Hauptbahnhof/ZOB – Berliner Allee – Sallstraße to Bahnhof Bismarckstraße, resurface on a ramp at Lindemannallee and continue to