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S21 is a term of the Berlin S-Bahn with several usages:
Contents

History

The two former services were always operated with 15 kV, 16.7 Hz AC overhead lines while all other S-Bahn lines in Berlin were and are operated with 800 V DC third rail. It was operated by Deutsche Bahn.
First operation in 2006
S21 service was introduced with the opening of the Berlin North-South mainline and the new Berlin Hauptbahnhof central station. DBAG Class 423 trains were used, these trains were lent from the Munich, Stuttgart and Frankfurt S-Bahns. The first S21 service was discontinued on July 11, 2006.
Second operation in 2009

In 2009 the S-Bahn service had to be strongly shortened due to lack of rolling stock. As a replacement and addition for the Nord-Süd-Tunnel-S-Bahn (S1, S2, S25 lines) the S21 ran again. Some S21 trains were extended to Hennigsdorf as a replacement for S25. This time trains of the Munich, Stuttgart and Rhine-Ruhr S-Bahns were used.
Future

The second north-south route (third rail) is planned since the 1930s. This route has the working title S21, but the route will not be exclusively served by the S21 line. A first part will be opened in 2017, connecting Berlin's central station with the northern part of the circle line. A S21 shuttle train will operate between Hauptbahnhof and Gesundbrunnen.

With the opening of the second section Hauptbahnhof – Potsdamer Platz around 2023, three S-Bahn lines are going to serve the new route: