use of air-operated doors had proved to be successful, modifications were made to enable a train to be operated by a crew of two. Between 1926 and 1928, the Central London cars were converted to air-door operation and it had been planned to convert the Gate Stock on the Piccadilly and Bakerloo lines. The cost of building a new car was only slightly more than the cost of converting an old one, and an order for new cars was placed. Delivered in 1929 and 1930, these vehicles enabled Gate Stock to be withdrawn from the Piccadilly line in June 1929. The last gate stock train ran on 1 January 1930. After 1929 the design was slightly altered with longer cars with wider doors and additional single-leaf doors on the trailers and electro-pneumatic brakes. Trains of this design were bought in 1931 and 1934 for the extensions of the Piccadilly line to South Harrow and Uxbridge, Hounslow and Cockfosters. London Passenger Transport Board The merger of London's underground railways, tramway companies and bus operators to form the London Passenger Transport Board (LTPB) was followed by the 1935–40 New Works Programme that included extension of the Bakerloo, Central and Northern lines. New trains were needed and four prototype six-car trains were built, three with streamlined cabs. Modern electrical equipment was fitted under the floor, removing the need for a control compartment on the motor cars, increasing the number of seats to 40. 1938 Stock, without the streamlining, entered service on the Northern line in June 1938. A seven-car train was formed from two trailer cars and five motor cars, one of which had no cab, the increased power giving greater acceleration. A total of 1,121 cars were built. By 1938 the Central line had been converted from the original 3-rail to the 4-rail system, the tunnels enlarged and platforms lengthened to take 8-car trains of the displaced Standard Stock, the last Central London Railway train running in 1939. In the same year Standard Stock replaced the 1904 Gate Stock on the isolated Northern City Line. The Bakerloo line received some 1938 Stock, these running with some converted trailers of Standard Stock. The Central line extensions were suspended during World War II and many Standard Stock cars were placed in store. These cars were in a poor state after the war and extensive refurbishment was needed when the extensions had been completed. After