The border between East and West Berlin was also significantly tightened, although not fully closed; East Germans were still able to cross into West Berlin, which then became the main route by which East Germans migrated to the West.[31] Between 1949 and the building of the Berlin Wall in 1961, an estimated 3.5 million East Germans – a sixth of the entire population – emigrated to the West, most via Berlin.[31] The railways between East Berlin and other important parts of East Germany went through West Berlin, so an easy way of going to West Berlin was leaving such a train.