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The first section of the Berlin U-Bahn to be built included an elevated section between Gleisdreieck and Potsdamer Platz stations. After the partition of Berlin, Gleisdreieck station was in West Berlin whilst Potsdamer Platz station was directly under the border to East Berlin. After the building of the Berlin Wall in 1961, the trains from both sides terminated at the last station before Potsdamer Platz (from the East: Mohrenstraße). Around 1972 also the two stations before Potsdamer Platz, on the western side, closed, because the area served by these stations was also served by another U-Bahn line.