assembled. A further 120 cars were ordered in 1925, with both types of equipment, to cope with the opening of the junction at Kennington and the lengthening of trains to six cars and from 1926, seven cars. Motor cars seated 30 passengers, while trailers had 48 seats. Control trailers had a cab at one end, but no switch compartment, and so seated 44. Trains initially had a crew of three, consisting of a motorman, a front conductor, and a rear guard. The guard signalled the conductor that the train was ready to go, and the conductor signalled the motorman. Once the use of air-operated doors had proved to be successful, modifications were made to enable a train to be operated by a crew of two. Changes included the fitting of a telephone, so that the guard and motorman could communicate, interlocks to ensure that all doors were closed, and the re-routing of the starting bell, so that it could be operated by a guard at the rear of the train, rather than the conductor at the front. These changes were completed by 1927, allowing the reduction of the train crew. 1926 and 1927 builds The superior nature of the new trains helped to show up the inadequacies of plans to convert large numbers of gate stock cars to have air doors, and they were largely abandoned. Another 112 cars, all fitted with GEC equipment, were ordered from Metropolitan Carriage Wagon and Finance in 1926. An order for 170 cars followed on in 1927. This enabled trains to be increased to seven cars, and more trains to be operated. A second order was placed with MCWF in 1927, for 136 more cars. The traction control equipment was supplied by British Thomson-Houston. Their equipment generally proved to be more reliable, and previous batches of Standard Stock were modified so that the main contactors could be replaced with BTH equipment. BTH continued to be the preferred supplier for this equipment for the next 35 years. Metropolitan-Vickers supplied their own motors. Standard Stock built by other manufacturers used GEC motors. The inter-operability of cars from several manufacturers with three types of traction control was the reason that they were called Standard Stock. The 1927 builds used a 'Z' bogie, instead of the previous 'Y' bogie, which had wheels of diameter, as opposed to . This required a modification to the WT54 motors, supplied by