The Ringbahn (German for: Circular railway) is a 37.5 km (23.3 mi) long railway line of the Berlin S-Bahn around the city centre of Berlin in Germany. The line is made up of the S-Bahn ring and the freight ring. S-Bahn service on the line is carried out by the S 41 (clockwise) and S 42 (counter-clockwise) circle lines, with 400,000 passengers a day. Due to its distinctive shape, the line is colloquially called "Hundekopf" (dog's head).
The Ringbahn also constitutes the outer border of the "A" zone for the Verkehrsverbund Berlin-Brandenburg transport association's fare structure, and is the outer border of the road traffic control zone for particulate matter control established by January 1 2008.
In 1851 the Berliner Verbindungsbahn, also known as the Königliche Bahnhofs-Verbindungsbahn or Royal Station Connection Railway, was completed between the terminal stations of the railways terminating in Berlin. This railway was laid directly in the streets of the urban area, disrupting traffic and also disturbing residents. In order to minimise the disruption of traffic, trains ran at night, with the train bell being constantly rung.
Therefore, plans were soon developed to build a new connecting line primarily for freight, which would run outside the then city limits. Funding for the construction was possible only after the victory in the war with Austria of 1866. Construction began in 1867 and the ring line was completed in 1877. The Lower Silesia-Mark Railway Company was commissioned to construct and manage the line.
The first section of the railway was opened on 17 July 1871 from the station of Moabit, through Gesundbrunnen, Central-Viehhof (now Storkower Straße), Stralau-Rummelsburg (now Ostkreuz), Rixdorf (now Neukölln) and Schöneberg (later Kolonnenstraße, now Julius-Leber-Brücke) to Potsdamer ring station, an annex to Potsdamer station. From there, trains returned in the opposite direction. This section into Potsdamer ring station became known as the Südringspitzkehre (Southern ring zigzag; this name reflected the need for trains to reverse there, eventually to continue their trip around the ring). The line crossed the Anhalt Railway (and later the Royal Prussian Military Railway) on bridges.
With the opening of the section from Schöneberg through the newly independent city of Charlottenburg (now Westend station) to Moabit on 15 November 1877 the ring was complete, with Potsdamer station still connected to the ring via the Südringspitzkehre for passenger trains.
From 1944 until the construction of the Berlin Wall in 1961, S-Bahn trains ran over the direct line between the stations of Papestraße (now Südkreuz) and Schöneberg opened in 1933, making a complete circle. With the building of the Wall the line was broken in two places:
- In West Berlin a separate line on a three-quarter ring ran between Gesundbrunnen and Sonnenallee or Köllnische Heide.
- In East Berlin the remaining section of the ring line ran between Schönhauser Allee and Treptower Park, running on the suburban lines to Bernau and Königs Wusterhausen or Schönefeld Airport.
After the 1980 S-Bahn strike, services on the western part of the ring was suspended for about 13 years.
On January 9, 1984, a treaty between East Germany and the West Berlin Senate entered into force and turned over the responsibility for operation of the S-Bahn in West Berlin to the West Berlin transport authority BVG. It was initially planned to restore the section between Westend and Sonnenallee.
After German reunification in 1990 plans were changed so that in 1993 the south ring was reopened to the junction with the line towards Baumschulenweg along with that line to allow a connection with the Goerlitz line. The reconstruction of the connection between Sonnenallee and Treptow Park required large-scale renovation that was not feasible in the short term. In the following years, the western part of the ring line has been put back into operation in stages. In 2002, the S-Bahn ring line was fully restored. Since May 2006, a full circular service has been operated as lines S41 (clockwise) and S42 (anticlockwise).
Starting from 1 January 1872 freight was carried out on the line to freight yards separate from the passenger stations. The operation of the ring line was electrified in 1926. In 1930, ring line operation were combined with the Stadtbahn and suburban services as the Berlin S-Bahn.
The building of the Berlin Wall in 1961 prevented continuous operation, after which passenger numbers on the West Berlin side, between Gesundbrunnen and Sonnenallee, continuously declined. This was caused partly by a politically motivated call for a boycott, because revenue from the West Berlin S-Bahn, which were operated by East German railways, supported the East German government. In contrast, the East Berlin section of the ring from Schönhauser Allee to Treptow Park remained in operation as it formed part of a major north-south tangent.
The S-Bahn ring line operations on the western part of the line closed in 1980 resumed on 17 December 1993 on the section between Baumschulenweg, Neukölln and Westend. Further sections were subsequently reopened: Westend–Jungfernheide on 15 April 1997; Neukölln–Treptow Park on 19 December 1997 and Jungfernheide–Westhafen on 19 December 1999. On 17 September 2001 the S-Bahn returned to the route over the old border between Schönhauser Allee and Gesundbrunnen.
It was not until more than twelve years after the fall of the Wall, that the last gap of the S-Bahn between Westhafen, Wedding and Gesundbrunnen was restored on 16 June 2002. Promotional material for the reopening referred to this as "Wedding Day", in allusion to the English word wedding. Services at that time did not immediately return to a continuous circle operation. For the time being, operated under the "screw concept”: trains entered the ring from the south at Neukölln and circled around it one and a half times until they ended at a station on the ring line. At the time the trip around the ring could not be achieved in less than 63 minutes.
Since 28 May 2006, the S-Bahn train have run on the ring line continuously. The trains take around 60 minutes with trains operating every five minutes in peak hour, and every ten minutes between the peaks and in the evenings. This is achieved through consistent use of the greatly accelerated 481/482 series trains. Some sections of the ring are used by other lines. On the southern ring coming from the Görlitz line in the southeast, S47 trains terminate at Hermannstraße, S46 trains terminate at Westend and S45 trains terminate at Berlin Südkreuz station, with some terminating at Bundesplatz. On the eastern section of the ring line S8, S85 and S9 trains operate between Schönhauser Allee and Treptower Park.
Under the so-called "mushroom concept" the long-distance lines on the northern part of the ring line for regional or long-distance services were rebuilt and electrified. On the ring line regional and mainline services now stop at Gesundbrunnen station and regional services stop at Jungfernheide.
The majority of the former ring line freight yards have been closed down or dismantled. Part of the former freight inner ring between Neukölln and Tempelhof is still used for freight transport and a freight depot still exists at Berlin-Moabit. The freight line is currently closed in the vicinities of Südkreuz and Ostkreuz.
Branches and connection curves 
Branches from the ring line have been built as follows:
- from Gesundbrunnen and Schönhauser Allee via Bornholmer Straße to Pankow and Schönholz (operating)
- from Treptow Park and Neukölln to Baumschulenweg (operating)
- from Jungfernheide via Wernerwerk to Gartenfeld (Siemens Railway, out of service and partially dismantled)
- from Jungfernheide via Siemensstadt-Fürstenbrunn to Spandau (S-Bahn tracks removed)
Connecting curves between the ring line and the Stadtbahn are at the Ostkreuz and Westkreuz stations.
- The south ring curve in Ostkreuz was until the evening of 28 August 2009 used by regular S-Bahn trains. It has been demolished and is being completely rebuilt. It was previously two tracks and is being rebuilt with two tracks.
- The north ring curve in Ostkreuz was closed down on 28 May 2006 and dismantled.
- A connection between Charlottenburg and Messe Nord/ICC (north ring curve) was used until 1944 and after destruction in World War II was not rebuilt.
- The connecting curve between Charlottenburg and Halensee (south ring curve) was rebuilt in the early 1990s with only one track. Currently it is used for service traffic and on weekdays by two services of line S46 each working day.
The Südringspitzkehre spur to Potsdamer Bahnhof was closed in 1944 due to war damage and never rebuilt. Its reconstruction is being considered in the planning options for line S21.
The following long-distance and freight curves have been built connecting to the ring line:
- from Berlin-Moabit, for freight from the west, formerly connecting to the Hamburger Bahnhof and Lehrter Bahnhof, still used for freight.
- in Wedding/Westhafen since 2006, connecting to the North-South mainline from both directions towards the Hauptbahnhof
- in Gesundbrunnen/Schönhauser Allee, connecting from both directions to the Stettin Railway and the northeast
- in Frankfurter Allee/Ostkreuz, from both directions to the Berlin Frankfurter Allee–Berlin-Rummelsburg line to Berlin-Lichtenberg station and the Rummelsburg marshalling yard
- in the Treptow Park area, from the north to the Görlitz line (currently out of service)
- in Neukölln, from the west to the Görlitz line
- in Hermannstraße, from the east to the Neukölln–Mittenwald line
- in Tempelhof/Südkreuz, a freight rail from the east to Berlin-Marienfelde (currently out of service)
- in Südkreuz/Schöneberg, a freight rail towards Zehlendorf (currently out of service)
- in Westkreuz/Halensee from both directions to the Wetzlar line
- in Westend/Jungfernheide from both directions to Spandau
- (German) "Rekordfahrgastzahlen bei der S-Bahn". Deutsche Bahn AG.
- (German) "AUS DER GESCHICHTE DER BERLINER RINGBAHN Der "Hundekopf" entsteht wieder". Berliner Zeitung. 2001-09-13. pp. p. S 07.
- Leo Favier, Aisha Ronniger Andrea Schulz, Alexander Schug, ed. (2009). Ring frei! Erkundungstour Ringbahn Berlin (Clear the ring! Exploring the Berlin Ring line) (in German). Berlin: Vergangenheitsverlag. ISBN 978-3-940621-04-7.
- Berliner S-Bahn Museum (2002). Strecke ohne Ende—Die Berliner Ringbahn (Line without end—The Berlin ring line) (in German). Berlin: Verlag GVE. ISBN 3-89218-074-1.
- Bienert, Michael; Hoppe, Ralph (2002). Eine Stunde Stadt (One hour city) (in German). Berlin: Berlin Edition. ISBN 3-8148-0096-6.
- Bley, Peter (1974). "50 Jahre Berliner S-Bahn (50 years of the Berlin S-Bahn)". Berliner Verkehrsblätter (in German) (21).
- Bley, Peter (1997). Die Berliner S-Bahn: Gesellschaftsgeschichte eines industriellen Verkehrsmittels (The Berlin S-Bahn: social history of a transport industry ) (in German). Düsseldorf: Alba.
- Bley, Peter (1980). Berliner S-Bahn: vom Dampfzug zur elektrischen Stadtschnellbahn (Berlin S-Bahn: from steam to electric rapid transport) (in German). Düsseldorf: Alba,.
- Suadicani, Waldemar (1915). "Berliner Ringbahn" (in German). Berlin, Wien: Röll: Enzyklopädie des Eisenbahnwesens.
- Pictures of the Ringbahn (German)