Because of the Piccadilly extensions, which were above ground, heaters were fitted, and the use of British materials in its construction was used for publicity purposes, at a time when there was an industrial depression. It became known as the 'All British' train, and operated on the Piccadilly and Northern lines. Transfer between the two was aided by the construction of a junction at Kings Cross in 1927. The Northern line offered heavier traffic, with which to assess the effectiveness of the new features. 62 production cars were ordered from Metropolitan-Cammell, which was formed from the Metropolitan Carriage Wagon and Finance Co. and Cammell Laird at this time, for delivery to the Bakerloo line. They replaced the Watford Joint Stock, which although relatively new, needed a larger crew and had to stop for longer at stations, because of its swing doors. Problems with the swing doors were highlighted by a fatal accident in which a passenger got caught in them, but the door interlock did not detect it. This build was not initially compatible with other builds, because it was fitted with an electro-pneumatic braking system, as well as the Westinghouse air brake. To accommodate its control, an extra 10-core control jumper was fitted. Previous builds of Standard Stock were gradually altered to include the extra wires, a process which was largely completed by 1936. Standard Stock motor cars which were running on the Bakerloo line with Cammell Laird 1920 Stock trailers were not fitted with the electro-pneumatic brake wiring until the trailers were withdrawn in 1938. 1931 and 1934 builds In 1929, government aid enabled plans for extensions to the Piccadilly line northwards to Cockfosters and westwards from Hammersmith to Acton Town, on tracks which were parallel to the District line but not shared with it, to come to fruition. The length of the line increased from to , and so more trains were needed. The improvements made for the 1930 build were incorporated into the design, as well as some new features. 145 motor cars were ordered from Metropolitan-Cammell, and 130 trailer cars were shared between two builders. All trailer cars had 12 doors, with two sets of double doors and a single-leaf door at both ends on each side. Electro-pneumatic brakes were fitted, as was a weak-field control, which enabled higher speed running and had been tested on the 1930 build. Its use on the Northern line