in 1900–1903. In addition, some of the displaced stock would be used on the Moorgate-Finsbury Park "Highbury Branch", also known as the Northern City Line, and some would be used to increase the length of trains on the Bakerloo line to seven cars. 82 control trailers were converted to trailers, and 21 driving motor cars were altered from "A" cars to "D" cars, as the makeup of 7 car trains on the Bakerloo and Central lines had the three-car unit at the opposite end to those on the Northern line. The Northern City Line had previously been served by surface stock, and had to be modified to accommodate the Standard Stock. Additionally, it had a non-standard arrangement of the third and fourth rails which supplied power to the cars, and these were modified to the same configuration as on the Northern line during the refurbishment. The use of control trailers at one end of a short train was virtually discontinued during the 1930s, because of the risks of disruption to services. A 3-car train consisted on a driving motor car, a trailer and a control trailer (DM-T-CT) and only had one compressor on the motor car. If it failed, the train was stranded, causing delays, whereas a 4-car short train had two motor cars (DM-T-T-DM) and hence two compressors. Failure of one did not prevent the train continuing. The exception to the use of control trailers was the off-peak trains on the Northern City Line, which consisted of a driving motor car and a control trailer, and these 2-car trains continued to run until October 1964. Central line On the Central line, Standard Stock began replacing the 1900/1903 Stock in the autumn of 1938, and this action was completed by mid-1939. The Central line used a three-rail system, with a central conductor rail supplying the power, and was not converted to a four-rail system until May 1940. All of the trains were therefore converted to work with the three-rail system as an interim measure, and then converted back to four-rail operation once the track had been modified. At the time, extensions to West Ruislip, Epping and Hainault were under construction, which would require more trains, but the onset of the Second World War resulted in the extensions being postponed. Nearly 200 cars were stored in the partially built Hainault depot, but that was then requisitioned by the U.S. Army Transportation