For a country of its size and degree of urbanization, Germany has surprisingly few cities with full-fleshed underground systems. Only Berlin, Hamburg, Munich and Nuremberg have real U-Bahn systems. There are several other cities having rapid transit pretender systems that may look like U-Bahn and have U designations, but they're rather cheap and dirty crossbreeds of undergrounds and streetcars/trams and therefore fall under the category of light rail systems.

• These so-called Stadtbahn systems run in Cologne (with Bonn), Frankfurt am Main, Stuttgart, Hannover, Bielefeld, several cities in the Ruhr and Karlsruhe^note noted for its tram-train system from 2020 on.
• Transit planner note: Using lighter vehicles enables running lines even on-street in the suburbs, which is a rather good way to make transit more accessible, but obviously won't work if high capacity is needed. This is why such lines are often "bundled" together in dedicated tunnels in the central parts of these systems.

Another example of cheap and dirty copies inside Germany are the S-Bahn systems.

• In Berlin and Hamburg, these suburban trains are bona fide heavy rail rapid transit, upgraded to work like a second U-Bahn in all but name long before the war, not only with separate tracks, but also with third rail instead of overhead wire.
• The typical copycat S-Bahn network is born by constructing a central city underground tunnel which will then be connected to existing railway lines that are supposed to feed the tunnel connecting the opposing ends, typically there's been one or two termini in the city and said tunnel made them partial through stations. That's what urbanrail.net dubbed as "S-Bahn with metro-like service". Poster childs are the systems for Frankfurt am Main, Stuttgart, Munich and the upcoming system for Leipzig, but Vienna and Zurich also fit the bill.
• Other cities had through stations from the beginning and therefore lacked the necessity, but also the chance, to construct a thoroughly new inner-city link for a suburban railway service.
• In some areas, an S-Bahn network can even be a merely rebranded regional rail. Needless to say, the standards of the various S-Bahn systems vary a lot.

Needless to say, both systems form a symbiotic relationship and Berlin's two rapid transit systems (U-Bahn (144 km) and S-Bahn (332 km)) combined would be the most expansive rapid transit network in the world, even surpassing the nominally longest systems in Beijing, Shanghai and The London Underground. That is, if the Germans weren't anal-retentive enough to differentiate. The systems are also run by different organizations, the U-Bahn by the BVG (the closest thing to a Berlin Transport Authority) and the S-Bahn by the DB (German Railways, successor of the German Federal Railways and German Imperial Railways).

Similar to The London Underground, the U-Bahn Berlin runs with two different profiles. The Kleinprofil (small profile) is the older one dating from before World War I with cars of a width of 2.30 m, accessing third rail from the side. The Großprofil (large profile) is the newer one with cars of a width of 2.65 m, accessing third rail from the bottom. Both systems however run on the same standard gauge of 1435 mm and the same voltage of 750 V DC. All cars are colored in school bus yellow, the corporate identity color of the BVG, just like all the city's trams and buses.

The S-Bahn cars of Berlin are also very particular. Beside the 3 meter car width and running on 750 V DC from a third rail instead of 15 kV AC from overhead wire as usual in S-Bahn trains elsewhere, they are bicolored in a goldenrod color in the upper half and red in the lower half (instead of just red).

The Berlin U-Bahn is an open system (i.e. no turnstiles) running from 5:00 to 1:00 every day and 24 hours on Friday and Saturday nights. It runs every 4-5 minutes at rush hour, 5-10 minutes off-peak and 15 minutes at nights when it runs. The S-Bahn Berlin runs from 4:00 to 1:00 and doesn't run a comparable night service to the U-Bahn's.

But even in the nights when rapid transit won't run, you can easily get by and around in most parts of Berlin via nightline buses (one-digited ones replace daytime subway equivalents) and the so-called "Metronetz" where buses and even trams run 24/7 on designated routes where they're supposed to offer rapid transit quality service where there isn't any.

For transportation purposes, Berlin and Brandenburg formed the VBB, their one and only transport association. The Berlin system in particular is divided into the three fare zones A, B and C. A is Berlin inside the Ringbahn (roughly "circle line"), B is Berlin outside it and C is for extensions into neighboring Brandenburg, primarily with the S-Bahn. Tickets are always offered for two zones at least, either AB, BC or ABC respectively. A one-way ticket e.g. would cost you € 2.40 in AB, € 2.80 in BC and € 3.10 in ABC.

The so-called Elevated And Underground (U-Bahn)

At the turn to the 20th century, Berlin like so many other cities had to deal with traffic problems. In 1902, the first line of the so called Hoch- und Untergrundbahn run by the private Hochbahngesellschaft (Elevated Corp.) was put into service. Actually, Siemens & Halske initially had their plans rejected and first put them into fruition in Budapest as a showcase. And as the name suggests, it wasn't completely underground, but rather elevated in (pre-1920 borders) Berlin with the exception of a stub to Potsdamer Platz and underground in the affluent Charlottenburg. Berlin was actually scared that an underground would hurt the recently built canals.

Until the start of World War I, the 2.30 m profile network of the Hochbahngesellschaft had three branches in the west (into Charlottenburg, Wilmersdorf/Dahlem and Schöneberg, lines later denoted with Roman numerals) and two in the east (Pankow and Friedrichshain, becoming A and B respectively), today they're the lines U1 to U4.

After said war and the expansion of Berlin in 1920, all new lines would be built to a 2.65 m profile. In the Weimar Republic, this would be the public Nordsüd-U-Bahn (later line C, today U6 and parts of U7), the GN-Bahn made by the AEG (later line D, today U8) and the line E (today U5) into eastern Berlin. After the BVG was established in 1929, the Hoch- und Untergrundbahn was shortened to the nice and perky U-Bahn, becoming the German catch-all term for underground, subway, metro, you name it.

The evolution of the S-Bahn

With the introduction of railways, the Prussian capital and primate city Berlin quickly had to deal with the consequences of congestion on its isolated rail lines that ended in several termini.

• To connect Berlin's various termini via then unsettled hinterland, the Ringbahn (ring/circle rail) was built and opened in 1871. Berlin would only later grow into the ring to make it an urban artery. It's dubbed Hundekopf (dog's head) due to its form and became an important technical boundary; beside being the outer limit of fare zone A and having an autobahn around half of it, it's also an official no-go area for dirty diesel cars.
• For a more passenger-oriented transfer possibility, the elevated, east-west running Stadtbahn (city rail) was built and opened in 1882. It was built from the start with separate double tracks for suburban and long-distance traffic. The system of Berlin's so-called Ring- und Vorortbahnen was then complete and became the "Stadt-, Ring- und Vorortbahnen''.
• Between 1890 and the turn of the century, the congested Vorortbahnen (suburban rails) and the Ringbahn got second sets of double tracks for a likewise functional separation.
• In 1924-30, the whole system underwent third rail electrification on par with the U-Bahn. On December 1, 1930, the system got rebranded as Stadtschnellbahn (city rapid railway) or S-Bahn for short, the mother of all other systems. ^note Curiously, the official "birthdate" of the S-Bahn is usually set in 1924 when the very first electrified single connection from Stettiner Bahnhof (today Nordbahnhof) to Bernau went online. The last tracks to Wannsee were converted in 1933.
• In 1934-39, the third and last of the S-Bahn core lines was built, the north-south running Nordsüd-S-Bahn-Tunnel or Nord-Süd-Tünnel in modern parlance, the only underground section of the whole system. Part of it had already been opened in 1936 for the Olympic Games. It was a direct consequence of a greater demand for this convenient electric rail system and acted as a precedent for what is understood as an S-Bahn: Deliberately digging a central underground tunnel to connect the lines from various termini or at least directions of a city. ^note If this sounds familiar, you're right. The RER of Paris, the Linee S of Milan, London's under-construction Crossrail and several other systems in the world are direct children of the German S-Bahn idea.

120 U-Bahn cars of large profile came to the Moscow Metro as a reparation for World War II. This was possible because the Soviets went to Berlin to learn how to make an underground run. Some of these cars were eventually bought back by East Berlin, most weren't. ^note On a related note, some older Berlin cars, so-called steel Doras, currently form the cornerstone of the Metro Pyongyang in North Korea. Same goes for 278 S-Bahn cars.

Needless to say, the division of Berlin seriously affected its infrastructure. From now on, West and East Berlin would diverge in their development. The BVG quickly divided into the BVG (West) and BVB (East).

West Berliners prefered to avoid traffic running through the east, so there rose a desire to create links that circumvented the East and later, when the Berlin Wall was erected, replaced the now boycotted S-Bahn. ^note Unlike the U-Bahn, being some kind of a better streetcar, the S-Bahn in Berlin was run as part of the DR (Deutsche Reichsbahn, German Imperial Railways) of the Soviet Zone (and later the GDR) as the Allies agreed that the Berlin railways were to be done by the Soviets as they surrounded the city anyway. Hence the GDR earned hard Western Deutschmarks with West Berlin S-Bahn guests while paying its bills in weak GDR marks. Not wanting to lose this concession, the GDR railways always kept the DR name instead of getting rebranded into "Bahn der DDR" or something like that. The boycott was the only undangerous way to effectively show dismay for the Berlin Wall without turning it into riots or even World War III, here by denying the GDR an income in West German currency. The courses of network extensions followed a so-called 200 kilometer plan that found its way into land-use planning in changing iterations.

Most important were two new lines: The north-south running line G (U9) opened quickly after the wall was erected and the southeastern line C I was expanded to become line H (U7) and came to thoroughly criss-cross West Berlin. Pre-war lines C (U6) and D (U8) were also extended, but they became more prominent for their middle sections becoming quintessential Sinister Subway lines. East Berlin stations of said lines were closed for the public and protected by armed guards and state security that became Geisterbahnhöfe, "ghost stations" or "haunted stations".

In 1958, West Berlin ceased to use its "letter + roman numeral" designation system for U-Bahn lines as they started to confuse the fellow pessenger, especially as the unbundling of the small profile lines led to designations up to "B IV". From now on, the letter designations have been used for internal purposes only and the lines were named for their mere termini. After the unbundling of the C lines in 1966, West Berlin introduced Arabic numerals to designate its lines, following the role model of Le Metropolitain. East Berlin only had two U-Bahn lines and therefore lacked the West's urgent need to seek clarity.

In the very end, the East Berlin line E (modern U5) was intended to receive a Western sister to access Tegel Airport and these were one day supposed to be merged into one big east-west line if political circumstances changed. Before that, a line U10 (formerly F) along the Wannseebahn was scheduled to be built from 1985 onwards, but quickly dropped when control of the Western S-Bahn was finally handed over. The reasons for this were, as usual, complicated.

The GDR already wanted to lease the S-Bahn in West Berlin to the BVG in the 1970s as the obligatory presence of the DR in West Berlin per Allied treaty proved to be expensive due to the boycott, but this offer was denied as West Berlin argued that it didn't belong to the DR, but the DR rather possessed mere operation rights. The S-Bahn continued to deteriorate without any attention, but this changed dramatically when the remaining employees for the DR in the West struck in 1980. The strike was quickly put down by Soviet authorities, but after widespread resignations from and dismissals by the DR and a drastic reduction of the schedule, West Berlin started to bother about what was left of the S-Bahn. It also helped that a political scandal rocked Berlin and led to snap elections in 1981. The conservative CDU under Richard von Weizsäcker won in a landslide, but failed to gain an absolute majority and formed a minority government whose support also depended on some rogue delegates from the liberal FDP.

On October 31, 1983, after the Allied powers and both German governments greenlighted a desired transfer, negotiations between the GDR-owned DR and the Senate of Berlin started and finished after two months; transfer of operation rights happened on January 9, 1984. Needless to say, a lot of renewal construction had to be done. They also attempted to change the livery of the trains to blue, but a massive public backlash forced them to go back to the traditional red and yellow.

In the East however, the S-Bahn became the backbone of public transport together with what remained of the U-Bahn and the tram/streetcar (which wasn't axed as it was in West Berlin until 1967 due to economic problems, even if so desired). The highest priority were housing projects and whatever kind of transport were to be used to access them was decided on the fly. In the case of Wartenberg and Ahrensfelde, S-Bahn was expanded, in the case of Hellersdorf, the line E (modern U5) of the U-Bahn was expanded with a generous overground section, like an ersatz S-Bahn. And all of them got tram access as well.

But just as the West, the East also had plans for new stretches of U-Bahn, the only difference is that the East couldn't afford it. The easternmost part of line F (Alexanderplatz - Weißensee) was scheduled to be built in the early 1970s and the GDR actually did precautions when it remodeled Alexanderplatz in the late 1960s, but eventually came to the conclusion that the country lacked the money to make it reality. Another mindchild of the GDR was a new U-Bahn line that current urban planning has designated as a line U11 along Mollstraße and Landsberger Straße. The GDR also wanted to reactivate lines C and D on its East Berlin stretches, cutting off their West Berlin ends from each other, but then the GDR died years before any efforts could come into fruition.

Hole In Flag

When the Berlin Wall was lifted on November 9, 1989, Bahnhof Friedrichstraße (the Tränenpalast or "palace of tears") quickly became overcrowded and alternatives had to be offered. While the borders were now open, custom controls were still officially required, so two former ghost stations of the line D (U8) with good access to either S-Bahn (Jannowitzbrücke, 11/11) or tram (Rosenthaler Platz, 12/22) were reopened as checkpoints and U-Bahn stations until the end of the year. The reactivation of the entirety of the remaining Geisterbahnhöfe only came on July 1, 1990, however, this being the date when the monetary and customs union came into effect. ^note or rather "economic, monetary and social union" between the two German states came into effect, making checkpoints inside of Germany obsolete Not too much later, Germany reunited in the same year. The co-reign of BVG and DR over the Berlin S-Bahn only ended with the start of 1994, when the old DB and the DR were united into the Deutsche Bahn AG or DB for short.

Since then, Berlin worked hard to extend minor stretches of U-Bahn to create useful links to the S-Bahn. And the S-Bahn of Berlin would now get thoroughly rebuilt with Western money. All main routes needed to be renewed to achieve the desire to rebuild the S-Bahn network to the quantity and quality of 1961, i.e. before the Berlin Wall. This was not achieved until 2002 when the Ringbahn was completely reopened again. The U-Bahn was considered re-unified in 1993 when East Berlin line A (U2) was reconnected to the remainder of the small profile network, discounting the 1995 reopening of Warschauer Brücke station after the reconstruction of the Oberbaum Bridge.

21st century - labor pains of a reborn metropole

After Hole In Flag reunited Germany and said Germany decided to make its old new capital live up to its name, Berlin turned into Europe's biggest construction site. On the other hand, Berlin gradually lost the special contributions it received from the federal government it got during the Cold War and which made up about half of its budget. Berlin from now on was integrated into Germany's inter-state fiscal adjustment and while this particular city-state profits most from it, it still feels the pain from adjusting from XXL to XL size. Especially after a banking scandal rocked the city in 2001, the budget situation has been abysmal.

First of all, it was decided to build a new central station at the site of old Lehrter Bahnhof. Berlin Hauptbahnhof was indeed opened in 2006, complete with a new railway tunnel crossing the Stadtbahn at said place. But the new Berlin railway node was of course far from complete. As the S-Bahn in Berlin needs separate tracks from the casual railways for reasons already mentioned, another north-south tunnel is currently under construction and its northern half scheduled to open in 2016, the project's name is S21.

Then there's the extension of the U5 from Alexanderplatz to Berlin Central Station via the historic centre of Berlin. A short stretch has already been opened between Central Station and Brandenburg Gate as an isolated stub called "U55", because the city would have had to pay back money to the federal government if they hadn't opened some kind of service within a certain time limit. The gap between U55 and U5 is currently under construction and the whole extension is scheduled to open in 2019.

Introduction of night services and the ascence of Captain Ersatz

Under the impression of the rush after the fall of the wall, the BVG started subway night line services in April 1990. Two selected lines crossing at Zoologischer Garten would from now on be served every 15 minutes in nights preceding workfree days.

• The north-south running U9 as a backbone of West Berlin and quickest way from East to West until broken lines were mended.^note How to go? Get from anywhere in East Berlin to the handiest former "ghost station", ride north into West Berlin to the next and only intersection with U9 (Leopoldplatz for U6, Osloer Straße for U8) and then ride straight southwest into City West. Another way would have been going south and change to U7, but U9 was especially designed to be quicker.
• The west-east running U1 (later U12 and eventually disbanded) from Ruhleben at the edge of Spandau to Kreuzberg ending at the Spree River.
• Finally in June 2003, the night grid was extended to most other subway lines, albeit with occasionally shorter routes. Notably lacking services in the night grid are U4 (stub line in Schöneberg), U55 (even shorter stub line and Butt Monkey of the republic) and the westernmost ends of U2 (Theodor-Heuss-Paltz), U3 (Thielplatz, near the Free University) and U7 (Jakob-Kaiser-Platz).

From December 2004, Captain Ersatz has reigned supreme with qualified bus and tramway services introduced as a so-called "metro network" or Metronetz with the products Metrobus and Metrotram. They're supposed to serve vital lines inside of Berlin that would theoretically deserve accessibility by rapid transit where there isn't any and from 2006, they've offered 24/7 services ever since.

Chaos in the S-Bahn

From winter 2008/09 onward, the S-Bahn Berlin has experienced its worst crisis since the 1980 strike. Maintenance was neglected as the DB tried to save costs on the wrong end to look good for a planned float on the stock market that eventually got scrapped due to the ongoing economic crisis, said neglection got so bad that the federal railway agency made the unprecedented move to forcably lay up a good chunk of the S-Bahn vehicle fleet. Contemporary witnesses are even quoted to say that the S-Bahn even ran better right after the war! Return to normalcy in operations is estimated to return in 2013.

The current contract between the city-state of Berlin and the DB will end with the schedule change of December 2017. To get the S-Bahn back on feet, Berlin wants to get serious about competition in its EU-wide call for bids that otherwise often end with the DB becoming the only bidder. Berlin divided its S-Bahn network into three parts (corresponding to its three core stretches) for this recent call of bids, starting the biggest part with the Ringbahn and the adjacent Görlitzer Bahn. Interested bidders came as far as from Britain, France and even China. There was some initial reluctance on the left side of the political spectre, rather condemning attempts for a "partial privatization" of public services, but the BVG rejected demands to take over the S-Bahn like it had under other political circumstances and it's not like the DB had acted like a bona fide public service provider.

Small profile lines

• U1: Color green. All iterations of line 1 were based on the former line B on the eastern stretches of the former Stammstrecke. In its modern iteration, it equals the course of the U15 in service for this first couple of years after Reunification. From 1961 to 1995, the eastern terminal Warschauer Straße was closed as it was its only station in East Berlin and the war-torn Oberbaum Bridge needed to be thoroughly rebuilt again.
• U12: The U12 is an episodic auxillary line that served as a festival and night line in 1993-2003 and gets reactivated whenever central portions of either U1 or U2 are broken to lower the number of transfer for normally not transfering passenger from two transfers to merely one. It actually matches the course of the line 1 from 1961 to 1993, fusing BI and AI, the latter now again part of the U2, the original line A.
• U15: The U15 was in service in 1993-2005 as an extension of the "old U3", formerly line BIV, a small stub line into Charlottenburg, into the eastern Stammstrecke. The stub was the result of a political horse trade wherein Charlottenburg reluctantly agreed to allow parts of a more important subway line (the modern U3) into rivaling Wilmersdorf to be built on its territory.
• U2: Color red. Started as the first truely underground subway section into Central Berlin from the western portions of the original Stammstrecke to form line A. Cut in half during the division of Berlin, its central overground section to the west of the Berlin Wall deteriorating and built over with a little maglev (the M-Bahn) shortly before Hole In Flag and disassembled just as quickly (1991) as it was assembled (1987). After 1966 until 1993, the line 2 or U2 used to be the name of what is now the U3.
• U3: Color lime green. Running all the way from Wittenbergplatz into the deep southwest of Berlin, the former lines AII and BII ran along the course that is now served by the U3. After unbundling, it's been internally dubbed as AII and also got the line number 2 after 1966, supposed to be merged with the line A in East Berlin one day, therefore following chronological order both in all of Berlin and West Berlin alone. After the central section of the modern U2 was rebuilt in 1993 and several lines were recombined, what is now the U3 became part of the U1. From 2005 on, Wittenbergplatz-Krumme Lanke became the U3. Besides serving many posh boroughs of Berlin, it also accesses the Free University of Berlin, one of the three Berlin university besides the old Humboldt University and the Technological University.
• U4: Color yellow. A short five-stop line running entirely in Schöneberg which used to be an own city when it decided to build the very first publically owned subway in Germany after the Hochbahngesellschaft deemed such a line not to be profitable enough. It already went online in 1910 and got the designation BIII when it was integrated into the Berlin network.

Large profile lines

• U5: Color brown. A small stub line in Charlottenburg labeled AIII was shortly the line 5, but was closed in 1970 and the number 5 was reserved for the line E in East Berlin that was built in 1930 from Alexanderplatz to Friedrichsfelde. It got a minor extension to Tierpark (East Berlin Zoo) in 1973 and a major extension into new housing projects in 1988/89. Currenctly, the middle section of an extension from Alexanderplatz to Berlin Central Station are underway. Theoretically, it should one day be continued to what is now Tegel Airport, but as said airport is due to be closed once Berlin Brandenburg Internatial Airport in Schönefeld opens and as Berlin is bankrupt, it will likely remain theory.
• U55: Opened in 2009, the U55 is an isolated stub of the U5 shuttling between Central Station and Brandenburg Gate. An alibi service in order to not pay back federal grants.
• U6: Color violet. With core sections built from 1912 on and completed in 1923, post-war extensions completed the line until 1966. Originally called the Nordsüd-Bahn due to its major direction, it was Berlin's first publically built subway line, also featuring the larger profile for the first time ever. Traditionally called line C and retaining the CII bifurcation after it finally split. Passed through East Berlin without stops expect for Bahnhof Friedrichstraße in 1961-89/90, being a border checkpoint during the city's division. Currently (2012-13) cut in half due to the construction of a new transfer station to the U5 called Unter den Linden.
• U7: Color light blue. Originally line CI and therefore considered the integral part of the Nordsüd-Bahn, first stretches were opened in 1924. After the war, the desire to unbundle bifurcations led to the re-conception of line CI into a southeast-northwest line H. After a one-stop extension and especially the reconstruction of Mehringdamm station from a bifurcation station to a transfer station, the C/CII tracks and the CI/H tracks got reopened as lines 6 and 7 in March 1966, introducing Arabic numerals for the U-Bahn lines. This was just an intermediate step, however, and the new line 7 was to see four major extensions in its west making it symptomatic for its Cold War nature.
  • In 1971, line 7 was extended up to Fehrbelliner Platz, crossing as many other lines as possible and making for a direct connection between Neukölln and City West, the CBD of West Berlin.
    • Until 1972, the southeastern extension of line 7 to Rudow was completed, accessing the housing project "Gropiusstadt" to the network and also featuring the newest depot of the Berlin U-Bahn.
  • Until 1978, a northwestern extension through Charlottenburg via Adenauerplatz (western end of the Ku'damm) to Richard-Wagner-Platz (near Charlottenburg City Hall) was constructed. Up to here, the route follows plans for a new circle line of the U-Bahn envisioned in the Albert Speer's Germania plans, a circle line for the U-Bahn inside the circle line of the S-Bahn which made the purpose of the line 7 fully clear: Providing an alternative to the S-Bahn.
  • In 1980, line 7 was extentended via Jungfernheide to Rohrdamm, accessing the Siemens works in Berlin. Some days before, the strike at the DR essentially castrated the S-Bahn in West Berlin and turned the line into a necessity.
  • In 1984, the U7 as it's been called now was extended to Spandau City Hall, bearing its total modern length and making it the world's longest tunnel for until 1988 when the Seikan tunnel opened.
• U8: Color blue. Initially called GN-Bahn as it runs from Gesundbrunnen to Neukölln, later line D. Initially built by the AEG and completed by the city of Berlin after the former's subsidiary went bankrupt and after it had its other subway done. First stretches were opened in 1927 and the initial core line was completed in 1930. Featured many ghosts stations in East Berlin that by now are under monumental protection for their 1920s style and experienced generous northward extension in the 1980s. Its southern terminus Hermannstraße served as an airraid shelter in World War II and traces of it were consciously worked into the final station design that was in all aspect two generations late.
• U9: Color orange. Built after the war as a bypass around East Berlin, accessing City West from the north and south. 20 days before it was supposed to open, the Berlin Wall was erected and the opening was preponed by five days. Considered the fastest of all U-Bahn lines in Berlin and also the first one to offer night services.

Phantom lines

• U10: Color black considered, formerly line F. In its historic conception, you could divide into the phases.
  • The maiden stretch Alexanderplatz - Weißensee was conceived in the 1920s and is part of all iterations. Alexanderplatz station was actually built in a way that one platform level would fit two east-west lines, the existing U5 on its inner tracks and the hypothetical U10/U3 on its outer tracks.
  • The central section Alexanderplatz - Kleistpark via Potsdamer Platz was originally planned as a westward extension of line E before World War II, but was quickly realigned to line F after the war and matched better to a clear direction.
  • The southwestern section (Steglitz - Kleistpark) was deliberately planned as a Department of Redundancy Department to the Wannseebahn serviced by the S-Bahn and therefore very vulnerable to any minor form of The Great Politics Mess-Up.
  • Nowadays the southwestern section is considered obsolete while the eastern section from Potsdamer Platz via Alexanderplatz to Weissensee (and even further) is still planned and a western extension would integrate a reconstructed version of the "old" U3 or former B IV (see U15), the line would eventually also be labeled as U3 should it ever go online, the "current" U3 getting recombined with the U1.
  • Students from the TU Berlin proposed a realization of the Cold-War link as a tram connection that isn't supposed to replace the S-Bahn, but rather to replace the current overcrowded bus service, most likely as an extended line M4.
• U11: As mentioned, a mind child of the GDR and officially planned, but far from realization, also due to excellent tram services on its route which are currently extended to Central Station just as said U-Bahn is supposed to be.

Modern S-Bahn numeration started in 1984 after the DR handed over the Western S-Bahn. S1 was reserved for the Wannseebahn that initially only ran until Anhalter Bahnhof, while the S2 passed all the Nord-Süd-S-Bahn tunnel in its course between Gesundbrunnen and Lichtenrade and the S3 ran west-east from Wannsee to Bahnhof Friedrichstraße where the two axes cross. The S4x family runs completely or partially on the Ringbahn. The Stadtbahn is entirely covered by lines with odd numbers, currently S5, S7 and S75 and formerly also S3. The bigger numbered lines S8, S85 and S9 pass the eastern Ringbahn, a heritage from East Berlin when this section was the backbone of their S-Bahn network.

(NS = Nord-Süd-S-Bahn-Tunnel; SB = Stadtbahn)

• S1: Wannsee - NS - Frohnau - Oranienburg
• S2: Blankenfelde - Lichtenrade - NS - Buch - Bernau
• S21: New north-south tunnel under construction to access Berlin Central Station
• S25: Henningsdorf - NS - Teltow
• S3: Ostkreuz - Erkner
• S4x: S41 and S42 are full circle lines in opposing directions, S45, S46 and S47 connect the southern Ringbahn with the Görlitzer Bahn.
• S5: Spandau - SB - Mahlsdorf - Stausberg (- Strausberg Nord)
• S6: Used to exist, but no longer does.
• S7: Potsdam - Wannsee - SB - Ahrensfelde
• S75: Westkreuz - SB - Lichtenberg - Wartenberg
• S8: Birkenwerder - Pankow - Ostkreuz - Grünau (- Zeuthen)
• S85: Waidmannslust - Ostkreuz - Schönweide (- Grünau)
• S9: Pankow - Ostkreuz - Schönefeld Airport