In the early 1930s, the London Underground were aware of the development of the metadyne equipment taking place at Metropolitan-Vickers, and the potential for regenerative braking which it provided. Before committing to an untried system, they therefore built a test train, by converting six cars originally built between 1904 and 1907 for the Metropolitan Railway. The work was carried out at Acton Works in 1934. Since a single metadyne unit could be used to control four motors, and each motor car had two motors, they were formed into two-car units, with a driving cab at the outer ends. By coupling the units together, trials of a two-car, four-car and six-car train could be carried out. The metadyne unit weighed about 3 tons, and consisted of three rotating machines, an exciter, a regulator and the actual metadyne machine, which were linked together mechanically. Electrically, the traction supply was fed into the machine, and the output fed the motors, without the need for starting resistances.