Northern and Bakerloo lines, and the need to order further stock to balance the fleet (the 1949 stock), cars were renumbered in the early 1950s. The DMs, trailers, as well as twenty-eight of the thirty NDMs had the '9' replaced by a '1', the DMs becoming 10324-10333, 11324-11333, the trailers 012389-012408, and the NDMs 12029-12054, 12056-12057. NDMs 92055 and 92058 were rebuilt into Uncoupling Non-Driving Motors (UNDMs) along with all twenty SNDMs. The twenty two cars rebuilt into UNDMs were renumbered 30000-30021. 1935/1949 Stock So successful was the 1938 Stock that, when, in 1948, additional cars were needed, 91 almost identical cars were built, 70 non-driving-motor (NDM) cars and 21 trailer cars. These were known as 1949 stock and operated with the 1938 stock. They were numbered in the same scheme; the UNDMs were numbered 30022-30045, 31000-31045 and the trailers 012495-012515. After World War II, the former 1935 stock streamlined DMs were rebuilt into trailers, and included with the 1938 stock, being renumbered 012477-012494. Before the war, three trailers were built for use with the streamlined DMs. These three cars differed from the 1938 trailers in that they were not equipped with compressors. However, the cars were not delivered until after the war and with the DMs rebuilt the three cars became part of the 1938 stock fleet, being numbered 012412-012414, and fitted with compressors. Technical Electrical system Each motor car is fitted with two series DC motors, one on each of the two bogies and are designed to operate from the 630 volt traction supply. Each motor has an output of approximately 250 horsepower. The controllers and switchgear are operated from a 50 volt auxiliary supply provided by a motor-generator set. This system allows the master controller at one end of the train to simultaneously operate the switchgear on all the motor cars in a similar manner to the system developed by Sprague for the Elevated railway in Chicago, Illinois some 46 years earlier. This same supply is also used to power the train lighting as well as some other auxiliary circuits. 'A' and 'D' motor cars The motor cars are divided into two distinct types, the 'A' end car and the 'D' end car. Each of the bogies fitted to the two cars is identical. This means that if the two motor cars at the two ends of a train were identical, they would attempt to pull the