An approx. 2.2 km long section of the U5 in the city centre of Berlin is currently under construction to close the gap between the existing underground structures at the Berliner Rotes Rathaus and Brandenburger Tor stations. Construction started early 2012 and the connection between Alexanderplatz and Brandenburger Tor is planned to open in 2019. The alignment starts from the newly built Rotes Rathaus Metro station, passing under the River Spree, the planned new Berliner Schloss and the Spree channel, and then follows the street Unter den Linden before arriving at the Brandenburger Tor. Three new stations have to be constructed at Rotes Rathaus, Museumsinsel and Unter den Linden, with the centrepiece being the Museumsinsel station where ground freezing techniques are being used.

The closing of the gap in line U5 of the Berlin underground completes the missing connection between the Alexanderplatz and Brandenburger Tor stations. The existing line U5 currently ends at the Alexanderplatz. With the closure of the gap, the existing tunnels at the Berliner Roten Rathaus will be connected to the Brandenburger Tor station on the line U55. The project includes the construction of three new underground stations and two single‐track tunnels connecting them, which are being tunnelling by shield machines. The new alignment thus passes deep under the city, under the River Spree, the new Berliner Schloss and the Spreekanal as well as running along beneath the street Unter den Linden to the Brandenburger Tor. The construction activities are taking place right in the centre of Berlin and the particular local conditions are a logistical and technical challenge.