West Berliners needed the hard-to get permits to enter East Germany by S-Bahn. With the construction of the Berlin Wall on 13 August 1961, all remaining railway traffic between West Berlin and its East German suburbs ended. Rail traffic between East and West Berlin was sharply reduced and restricted to a small number of checkpoints under GDR control. East Berliners and East Germans were then unable to freely enter and leave West Berlin. However, international visitors could obtain visas for East Berlin upon crossing one of the checkpoints at the Wall. Following the policy of détente of the Federal Government under Chancellor Willy Brandt, West Berliners could again apply for visas to visit East Germany, which were granted more freely than in the period until 1961. On 4 June 1972, West Berlin's public transport operator BVG could open its first bus line into the East German suburbs since 1950 (line E to Potsdam via Checkpoint Bravo as it was known to the US military). This route was open only to persons bearing all the necessary East German permits and visas. For visits to East Germany, West Berliners could use four checkpoints along the East German border around West Berlin: The two road transit checkpoints Dreilinden (W)/Drewitz (E) and Berlin-Heiligensee (W)/Stolpe (E) as well as the old transit checkpoint at Heerstraße (W)/Staaken (E) and the checkpoint at Waltersdorfer Chaussee (W)/Schönefeld (E), which was also open for travellers boarding international flights at Schönefeld Airport. Traffic between East and West Berlin While East and West Berlin became formally separate jurisdictions in September 1948, and while there were travel restrictions in all other directions for more than a decade, freedom of movement existed between the western sectors and the eastern sector of the city. However, time and again Soviet and later East German authorities imposed temporary restrictions for certain persons, certain routes, and certain means of transport. Gradually the eastern authorities disconnected and separated the two parts of the city. While the Soviets blocked all transport to West Berlin (Berlin Blockade between 24 June 1948 to 12 May 1949), they increased food supplies in East Berlin in order to gain the compliance of West Berliners who at that time still had free access to East Berlin. West Berliners buying food in East Berlin were regarded as approving of the Soviet attempt to get rid of the Western Allies in West Berlin. This was seen