started operations between Billstedt and HafenCity University, sharing most of its route with the U2 and thereby increasing service on the highly used line between Berliner Tor and Billstedt. The U4 was extended further east to Elbbrücken in December 2018, providing a future S-Bahn connection. Future development Abandoned projects The eastern branch of the "Walddörferbahn" was originally intended to terminate at the station Beimoor, just to the north of Großhansdorf, in order to connect a proposed housing development to the public transport system. By the time work was abandoned, the station platform had already been finished and tracks had been laid. Today the remains of the station are used as a winter home for bats in the middle of a dry grass biotope. Plans for a fourth line have been discussed since the 1970s. In those days it was planned for the line to lead from City Nord and Uhlenhorst via the city centre (Hauptbahnhof Nord and Jungfernstieg) and Altona to Lurup and Osdorfer Born. In order to link the green-field housing development at Osdorfer Born to the rest of the city as quickly as possible, a limited service on the stretch between Altona und Osdorfer Born was planned. Until the connection with the rest of the network was ready, it was intended to transport trains to the main service depot in Barmbek by flatbed trucks for maintenance. Platforms and track beds for this stretch were completed in the stations Hauptbahnhof Nord and Jungfernstieg. Preparations for tunnel constructions were made in the area of Hamburg-Altona railway station and under the motorway A 7 in Bahrenfeld. In preparation for the junction with the U1, an extra platform was built at the new station Sengelmannstraße. All these constructions remain unused to this day. The northern surplus platform area at Hauptbahnhof Nord is currently used for the art installation Sterne by Raimund Kummer and Stefan Huber. Here several concrete stars lie over the whole platform area as if they had fallen from the sky. At the same time the southern surplus platform area is still unused in any kind of way in 2018, displaying the station as it was opened in 1968 since changes in terms of advertising spaces, station signs etc. were never applied there. However, none of those two unused platforms are accessible as they are locked with fences. Today both unused platforms appear deteriorated and large segments of the wall