The idea for a North-South Link rapid transit rail line from Unter den Linden to Yorckstraße, via Potsdamer Platz and Anhalter Bahnhof, had first been mooted in 1914, but it was not planned in detail until 1928, and then approval had to wait until 1933.

In spite of all the setbacks, it was opened from Unter den Linden to Potsdamer Platz on 15 April 1939, extended to Anhalter Bahnhof on 9 October, and then to Yorckstraße, to complete the link, on 6 November.

Four platforms were provided at the station, and all were used although just two were planned to suffice: the other two were originally intended to be utilised by another new line, which was to branch off eastwards and run under the city to Görlitzer Bahnhof.

The North-South Link, less than six years old, became the setting for one of the most contentious episodes of the final Battle for Berlin, in late April and early May 1945.

Shortly after the war's end, the Ringbahnhof got a reprieve of sorts, temporarily reopening on 6 August 1945 as the terminus of the Wannseebahn trains, while the Nord-Süd-Tunnel received massive repairs (millions of gallons of water had to be pumped out).

This gave rise to the infamous "Geisterbahnhöfe" (ghost stations), Potsdamer Platz being the most notorious, which were sealed off from the outside world and trains ran straight through without stopping.

They would generally slow down, however, affording passengers the strange sight of dusty, dimly lit platforms patrolled by armed guards, there to prevent any East Berliners from trying to escape to the West by train.

At the points where the lines passed directly beneath the actual border, concrete "collars" were constructed within the tunnels with just the minimum clearance for trains, to prevent people from clinging to the sides or roof of the coaches.

The U-Bahn, or Untergrundbahn (underground railway), was a major revolution in Berlin's public transport, and the forerunner of similar systems now seen in several German cities.

Though it was reached via an entrance right outside the main-line terminus, people then had to walk about 200 metres (660 ft) along an underground passage beneath the appropriately named Bahnstraße (Railway Street).

The enormous Wertheim Department Store in nearby Leipziger Straße did enjoy such an entrance, as in later years did the Hotel Excelsior from the Anhalter Bahnhof.

This was partly the case with the U-Bahn line through Potsdamer Platz, as in October 1991, the Mohrenstraße station operationally became a terminus for trains on the eastern side.

In the latter years of the Wall's existence, part of the abandoned U-Bahn section, the stretch between Gleisdreieck and Potsdamer Platz, was used by the M-Bahn (Magnetic Levitation Railway).

As early as the late 1970s the West Berlin government had discussed introducing such a system to the city, particularly a section linking Tegel Airport with the centre.

The go-ahead was finally given for the building of a test track at Potsdamer Platz on 2 December 1980, with a ground-breaking ceremony taking place on 16 June 1983.

On 18 April 1987 an arson attack at Gleisdreieck destroyed two cars, while a more spectacular mishap occurred on 19 December 1988 when a train with badly adjusted brakes ran through the end wall of the Kemperplatz terminus, much to the amusement of the local press.

Traffic forecasts before the opening assumed 80,000 passengers per day, including 50,000 exiting or entering and around 30,000 people transferring to the underground and S-Bahn.