In 1949, the flat-fronted units were modified to run as two-car shuttle trains, and were to be transferred to the Central Line. Some of the streamlined cars had been through an overhaul process, where the bogies had been upgraded to conform to 1938 Stock standards. As the flat-fronted units had not received an overhaul, they were given bogies removed from the updated streamlined cars. The Wedglock couplers at the outer ends were removed and replaced by Ward couplers, so that they could be mechanically coupled to Standard Stock trains, and air hoses were fitted. The Metropolitan Vickers control system was removed, and replaced with the British Thomson-Houston equipment, which had been removed when those units were converted to trailers. The guard's control panel, which had been located in the cab, was moved to the trailing end of the 'D' cars, so that the guard remained in the same position whichever the direction of travel. To ensure that each train ran with two compressors, KLL4-type compressors were fitted to all of the cars.