The first train to enter passenger carrying service did so on 8 April 1937 on the Piccadilly Line, and the final one followed nearly a year later, on 10 March 1938. Initially, they ran as four-car trains during the daytime, avoiding the morning and evening peak services. From 26 April 1937, an extra two-car unit was added to the four-car trains at the end of the afternoon service, and the six-car formation run through the evening peak. Four cars of flat-fronted stock began running from 24 January 1938, but cars 10009, 11009, 10010 and 11010 were transferred to Golders Green depot on the Northern Line on 31 March 1938, so that gauging tests for the 1938 Stock could be carried out. They returned to the Piccadilly Line on 4 July 1938.