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.