Three seven-car trains of 1956 stock were built, each train by a different manufacturer. The companies involved were Birmingham RC&W, Gloucester RC&W and Metro-Cammell, although the units supplied were of near-identical external appearance, and fitted with the same equipment. Each company provided a three-car and a four-car unit, the three-car unit consisting of a trailer car with a driving motor car at both ends, and the four-car units being of similar formation, but with an additional non-driving motor car in the rake.