The United States responded with the Berlin Airlift, dropping supplies into Berlin via cargo planes to maintain the western portion of the city as separate from Soviet control.
When the British, French and American authorities combined their zones of control into a unified West Germany and West Berlin, the Soviets established what has become known as the Berlin Blockade -- the closure of the various transportation routes you describe. By the spring of 1949, it was clear that the airlift strategy had worked and the Soviets gave up on the blockade.
