 The batch of 9 vehicles supplied by Gloucester Railway Carriage and Wagon Company between 1936 and 1938 set the standard for subsequent builds.Including this batch, 52 machines had been built by 1986, in six batches from four manufacturers, with one built at London Transport's Acton Works.Each new batch included some improvements, but most used electro-pneumatic traction control equipment made by GEC, and so could be operated together.