holders of Western Deutsche Marks could buy as much they could afford, up to five and six east marks were offered for one west mark. In the East, however, the Soviets had arbitrarily decreed a rate of 1 for 1 and exchanging at other rates was criminalised. On 12 May 1949, the Blockade ended and all roadblocks and checkpoints between East and West Berlin were removed. The Berlin Airlift, however, continued until 30 September 1949 in order to build up supplies in West Berlin (the so-called Senate Reserve), in readiness for another possible blockade, thus ensuring that an airlift could then be restarted with ease. On 2 May 1949 power stations in East Berlin started again to supply West Berlin with sufficient electricity. Before then, electrical supply had been reduced to just a few hours a day after the normal supplies had been interrupted at the start of the Blockade. However, the Western Allies and the West Berlin City Council decided to be self-sufficient in terms of electricity generation capacity, to be independent of Eastern supplies and not to be held to ransom by the eastern authorities. On 1 December 1949 the new power station West (, in 1953 renamed after the former Governing Mayor of West Berlin into Kraftwerk Reuter West) went online and West Berlin's electricity board declared independence from Eastern supplies. However, for a time Eastern electricity continued to be supplied albeit intermittently. Supply was interrupted from 1 July until the end of 1950 and then started again until 4 March 1952, when the East finally switched it off. From then on West Berlin turned into an 'electricity island' within a pan-European electricity grid that had developed from the 1920s, because electricity transfers between East and West Germany never fully ceased. The 'electricity island' situation was noticed most in situations of particularly high demand; in other areas of Europe peaks in demand could be met by tapping into electricity supplies from neighbouring areas, but in West Berlin this was not an option and for certain users the lights would go out. In 1952 West Berliners were restricted entry to East Germany proper by means of a hard-to-obtain East German permit. Free entry to East Berlin remained possible until 1961 and the building of the Wall. Berlin's underground (Untergrundbahn, U-Bahn) and Berlin's S-Bahn (a metropolitan public transit network), rebuilt after the war, continued to span all occupation sectors.