The line will be built in sections. In 2005, the zoning approval for the northern part of the route from the Ringbahn to Hauptbahnhof was adopted. In October 2009 a loan agreement was entered into between the Senate and Deutsche Bahn for the first section. This provided for total costs of €226.5 million. 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.