The Central London Railway Stock were electric multiple units composed of trailers that had been converted from carriages designed to be hauled by electric locomotives with new motor cars. The Central London Railway opened in 1900 with electric locomotives hauling wooden carriages, but the heavy locomotives caused vibrations that could be felt in the buildings above the route. Following investigation it was found that conversion to electric multiple units solved the problem, so new motor cars were bought and replaced all the locomotives by June 1903. Trains normally ran with six-cars, four trailers and two motor-cars, although some trailers were equipped with control equipment to allow trains to be formed with 3 cars.