 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.