 By the early 1950s, this was a serious disadvantage, when a series of failures occurred, which required extensive repairs.A decision was taken to remove the equipment and replace it with a Pneumatic Cam Motor (PCM) system, using spare controllers from the 1938 tube stock.The first converted train entered service on 31 March 1955, and the stock was redesignated at CO/CP Stock, since it contained cars from both batches.