The Berlin outer ring is a 125 km (78 mi) long double track electrified railway, originally built by the German Democratic Republic to bypass West Berlin in preparation for the building of the Berlin Wall during the division of Germany. It was developed by East Germany for economic, transport policy, and military reasons between 1951 and 1961 and included parts of some older lines.

West Berlin was a political enclave which comprised the western part of Berlin from 1948 until 1990, during the Cold War. Although West Berlin lacked any sovereignty and was under military occupation until German reunification in 1990, the territory was claimed by the Federal Republic of Germany (FRG), despite being entirely surrounded by East Germany (GDR). The legality of this claim was contested by the Soviet Union and other Eastern Bloc countries. However, West Berlin de facto aligned itself politically with the FRG from May 1949 and was thereafter treated as a de facto city-state of that country. After 1949, it was directly or indirectly represented in the institutions of the FRG, and most of its residents were citizens of the FRG.