Following the conclusion of the Potsdam Conference, what was formerly greater Berlin was divided into four sectors, administered jointly by the Allied powers (the United States, Great Britain, France, and the Soviet Union). This map shows the boundaries of the four sectors, which remained formally in place until 1990. After the building of the Berlin Wall in 1961, however, it became increasingly clear that the division was purely one of east-west. Within West Berlin, there were hardly any dividing lines between the sectors administered by the Western Allies. Berlin had unified transportation systems, including railroads and the S-Bahn [suburban railway], which still existed in 1949. It was not until 1961 that railroad and rapid transit lines were interrupted by the construction of the Berlin Wall.