The various types of streamlined units were used interchangeably to make up four-car and six-car trains, but the flat-fronted units were kept together. There were significant teething problems with the new equipment, and when the outbreak of World War II made maintenance difficult, the trains were put into storage at Cockfosters depot. Three of the cars found temporary use as air raid precaution (ARP) shelters, one at Northfields depot, and two at Cockfosters. They were parked over a pit and surrounded by sandbags, for use in an emergency. They found no further use in service until the end of 1948, when there were plans to remove the streamlining and drivers cabs from the streamlined cars, and for them to become trailer cars which would run in 1938 Stock trains. Work began on the first conversion in early 1950, and was quite extensive. In addition to the rebuilding of the bodywork, all of the traction equipment was removed, and a complete rewire of the electrical system was needed. When the work was completed, the trailers looked similar to the 1938 Stock, but there were some visible differences. The converted cars only had three windows between the single end door and the first set of double doors, whereas the 1938 Stock had four, and the single door itself was further from the end of the car. Trailer number 012484 was outshopped in August 1950, and entered service on the Northern Line in September. It was monitored to see how it performed, and the remaining 17 conversions incorporated changes to the doors and bogies as a result. The motor cars became trailers 012477 to 012494, and the last one to be used in passenger service was number 012489, which was withdrawn in 1976.