When the railway opened in 1900, it was operated by electric locomotives hauling carriages with passengers boarding via lattice gates at each end. The locomotives had a large unsprung mass, which caused vibrations that could be felt in the buildings above the route. After an investigation by the Board of Trade, by 1903 the carriages had been adapted to run as trailers and formed with new motor cars into electric multiple units. The Central London Railway trains normally ran with six cars, four trailers and two motor-cars, although some trailers were later equipped with control equipment to allow trains to be formed with 3 cars. Work started in 1912 on an extension to Ealing Broadway, and new more powerful motor-cars were ordered. These arrived in 1915, but completion of the extension was delayed due to the outbreak of World War I, and the cars stored. In 1917 they were lent to the Bakerloo line, where they ran on the newly opened extension to Watford Junction. Returning in 1920/21 and formed with trailers converted from the original carriages they became the Ealing Stock. In 1925–28 the trains were rebuilt, replacing the gated ends with air-operated doors, allowing the number of guards to be reduced to two. After reconstruction of the Central London Railway tunnels, the trains were replaced by Standard Stock transferred from other lines and the last of the original trains ran in service in 1939.