The third unit, consisting of cars 10011 and 11011, had been moved to Ealing Common depot in early 1957, to be used in a trial of regenerative braking. The concept, which involves the motors being used as generators, and current being fed back into the system for use by other trains, had been tried previously, on the O and P surface stock trains, which were equipped with metadyne controllers. The metadyne controllers were being replaced at the time, as they were proving increasingly unreliable, but London Underground still hoped that the system could be used to reduce energy costs. The 1935 Stock train was fitted regenerative braking equipment manufactured by British Thomson-Houston, and the substation at Northfields was adapted to cope with the regenerated current. Because it was a trial system, the equipment was installed in the passenger area, and a self-lapping switch was fitted to the brake controller, which tilted as the brakes were applied. Testing of the system ran from May 1957 to early 1958, when a system manufactured by General Electric Co replaced it, and a third set of tests used equipment by Metropolitan Vickers. Although regenerative braking did not become part of the 1959 Stock or 1962 Stock production runs, the British Thomson-Houston equipment was modified to allow tests of rheostatic braking on a test train of 1960 Stock, which paved the way for it to be incorporated into the 1967 Stock built for the newly constructed Victoria Line.