The main production run of O Stock consisted of 116 motor cars, which were formed into 58 two-car units. Trials began with a four-car formation on the District line between High Street Kensington and Putney Bridge in September 1937, and a six-car formation began working on the Hammersmith line in January 1938. There were some technical problems, caused by the demands put on the power supply system when a train of six motor cars started, and the amount of power that such a train attempted to return to the system when the regenerative brakes were used. This was partially mitigated by ordering a further 58 trailer cars, and converting each two-car unit into a three-car unit, by inserting a trailer car into the formation. A batch of P Stock was then ordered, to replace the trains on the Metropolitan line. Although O and P Stock units could be coupled together, the metadyne units in particular were not the same, and could not be interchanged between the builds. 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. All of the metadyne equipment was subsequently replaced.