On 27 November 2009, the preparatory work for this phase of construction started at the Hauptbahnhof. For the underground excavation in Invalidenstrasse, diaphragm walls were built into the ground and the trench in between was covered with a reinforced concrete lid. This stage involves the construction of a curve to the Westhafen and an eastern connection to Wedding inside the northern Ringbahn. Structural preparation for these junctions to these lines had already been made during the construction of the North–South mainline in 2006. From there, the existing line will run in a southerly direction (in the tunnel layer) to the Berlin Hauptbahnhof east of the North–South mainline. The realization of an intermediate station under the working name of "Perleberger Brücke" (as a two-level station in a V-shape) is provided as an option. It is proposed to build this 1,600-meter-long section by 2016. The construction of the second section of the S21 is to begin no earlier than 2018 and is expected to be completed in 2023. The new S-Bahn line will run in a tunnel from near the Hauptbahnhof past the Reichstag to Potsdamer Platz. It will run next to the existing Nord-Süd Tunnel to Brandenburg Gate and separate from it to run to Potsdamer Platz. The first north-south S-Bahn tunnel was designed in 1939 with room for an additional two tracks at Potsdamer Platz and to its south for the new line. The cost of the S-Bahn line (phases 1 and 2) has been estimated at €317 million (2009 prices). The benefits of additional expenditure to the east of the Reichstag are still under investigation. This would increase the cost to about €330 million. There are currently no dates set for the other phases of construction to the southern Ringbahn. It has so far only been defined in the Berlin land use plan. Since reunification, there have been suggestions that lines that have not been used since 1961 or 1980 should be rebuilt and connected to the network by some new lines. Many of these plans have changed several times since then or have been abandoned. Following a decision of the Berlin House of Representatives, the goal is essentially to restore the S-Bahn network to its extent in 1961. This was stated in an agreement between Deutsche Bahn, the Federal Ministry of Transport and the Senate on 4 November 1993.