Berlin's circular S-Bahn line around all of the Western and Eastern inner city. The lines were separated and those mostly located in West Berlin were continued, but only accessible from West Berlin with all access in East Berlin closed. However, even before the Wall had been built, West Berliners increasingly refrained from using the S-Bahn, since boycotts against it were issued, the argument being that every S-Bahn ticket bought provided the GDR government with valuable Western Deutsche Marks. Usage dropped further as the Western public transport operator BVG (West) offered parallel bus lines and expanded its network of underground lines. After the construction of the Wall, ridership fell so much that running the S-Bahn lines in West Berlin turned into a loss-making exercise: wages and maintenance – however badly it was carried out – cost more than income from ticket sales. Finally, the Reichsbahn agreed to surrender operation of the S-Bahn in West Berlin, as had been determined by all Allies in 1945, and on 29 December 1983 the Allies, the Senate of Berlin (West; i.e. the city state government) and the Reichsbahn signed an agreement to change the operator from Reichsbahn to BVG (West) which took effect on 9 January 1984. On 9 November 1989 East Germany opened the borders for East Germans and East Berliners, who could then freely enter West Berlin. West Berlin itself had never restricted their entry. For West Berliners and West Germans the opening of the border for free entry lasted longer. The regulation concerning one-day-visas on entering the East and the compulsory minimum exchange of 25 Western Deutsche Marks by 1989, continued. However, more checkpoints were opened. Finally, on 22 December 1989, East Germany granted West Berliners and West Germans free entry without charge at the existing checkpoints, demanding only valid papers. Eastern controls were slowly eased into spot checks and finally abolished on 30 June 1990, the day East and West introduced the union concerning currency, economy and social security (). Traffic between different parts of West Berlin crossing the East When the Wall was built in 1961, three metro lines starting in northern parts of West Berlin passed through tunnels under the Eastern city centre and ended again in southern parts of West Berlin. The lines concerned were today's underground lines U 6 and U 8 and the S-Bahn line S 2 (today partly also used by other lines). On the