Construction On 19 January 2000, construction began on the western end of this extension. However, around that time the city council suffered a major financial crisis. The city had accepted money from the German federal government for the construction work already finished, and by the terms of the agreement, the city would have had to return the money if there were no operating trains on the line. In 2004, the city and federal governments reached a compromise: the city would complete the short section of line that was largely complete between Berlin Hauptbahnhof and Unter den Linden S-Bahn station (renamed "Brandenburger Tor" in 2009) and run it as a single-track shuttle with a single train, without any signaling. Although transit planners projected that such a shuttle would not attract a significant ridership, the city determined that the cost of building and operating the line would be less than the cost of returning the money to the federal government. Opening The opening of the line between Berlin Hauptbahnhof and Brandenburger Tor was delayed owing to extreme groundwater problems at the site of the latter new station. It was finally opened on 8 August 2009. Operation Stations With the opening of the U-Bahn station, the existing Unter den Linden S-Bahn station was renamed Brandenburger Tor (for the nearby Brandenburg Gate). The name "Unter den Linden" was then given to a new station further east at the junction with Friedrichstraße, where the U5 intersects the U6. The train ran every 10 minutes and, due to the low demand, there was no night service. Rolling stock Because the U55 was not physically connected to the rest of the U-Bahn system, any trains being delivered to the new line had to be taken by low loaders on the street and set on track through a tunnel opening located north of Hauptbahnhof station. There was also a provisional workshop for the basic maintenance and cleaning of carriages. Formerly, trains running on the U55 were F79 trains. However, due to a lack of trains on other large profile U-Bahn lines, the two U55 trains were removed and placed on the U6 in April 2017. Instead, an older D class train was renovated and placed into service until March 2020. Linking U55 and U5 The designation of the line as U55 indicated that it was ultimately intended to become part of the U5 line. Construction of the link