Hitler visited the frontlines by using either aeroplane or his special train, the Führersonderzug; thus, the Führersonderzug can be considered as the first of his field headquarters. The first permanent installation which became a Führer Headquarters was the Felsennest, which was used by Hitler during the Battle of France in May, 1940. Hitler actually spent very little time in Berlin during the war, and the dwellings he most frequently used were the Berghof and the Wolfsschanze, spending more than 800 days at the latter.