Berlin - The city will get its newest U-Bahn Friday when the U5 is extended from Alexanderplatz to Hauptbahnhof after nearly a decade of construction and about €525m. The first subway cars will begin running through the 2.2km extension's twin tunnels at 12pm.

 "The U5 will bring necessary relief for the city's railways," Jens Wieseke, deputy head of local rail interest group Igep, told newswire dpa. The new line will help remove passenger pressure on the central railway viaduct through Mitte that carries S-Bahns as well as regional and long-distance trains.

The extension was to have opened in 2017 but construction through Mitte proved slower than expected – archaeologists discovered foundations from various villas as well as the former Rotes Rathaus, leading to changes in exits at the city hall station. A 700-tonne tunnel-boring machine was used to drill the twin tunnels under Mitte.

Construction, which originally started in 1998 amid the reunification euphoria of the time but was later halted, also included adding the stations Rotes Rathaus, Museumsinsel and Unter den Linden. The Bundestag and Brandenburger Tor stations were completed in 2009 and opened as the temporary U55, which shuttled shortened trains between Hauptbahnhof and Berlin's trademark gate. 

But not all is perfect. The Museumsinsel stop isn't yet complete and won't open until summer and the U6 stop Französische Straße will be closed because it is too close to the Unter den Linden station. The construction also sparked changes in Spree ship traffic that required the construction of a special harbor between city hall and the Karl Liebknecht bridge, adjacent to the Dom.

BVG picked Friday because it's Saint Barbara day. She's the patron saint of miners and tunnelmakers.