O Stock trains were built for the Hammersmith & City line, using metadyne control equipment with regenerative braking, but the trains were made up entirely of motor cars and this caused technical problems with the traction supply so trailer cars were added.

[1] As part of the 1935–40 New Works Programme, the O stock, sets of two motor cars, was built for the Hammersmith & City line.

The Gloucester Railway Carriage and Wagon Company (GRC&W) and the Birmingham Railway Carriage and Wagon Company (BRC&W) built 116 cars, allowing 19 six-car trains and a spare two-car set.

The first reformed train went into service on 18 July 1938,[3] and 19 O Stock sets were transferred to the Metropolitan line.

[4] The 51-foot (16 m) long O Stock cars were fitted with air operated doors under the control of the guard.

Externally, the cars had side panels that flared at the bottom with no running boards to prevent passengers jumping on a moving train and attempting to open a door.

[9] The trailers, with four extra seats,[3] had a hinged door at one end, locked closed in normal use, so they could be converted into driving motor cars.

[4][12] Because of the reliability of the metadynes and difficulties repairing them, one Circle line train was converted to use the pneumatic camshaft control (PCM) equipment used on the 1938 tube stock and entered service in March 1955.

This was considered a success and the other Circle line trains were converted, followed by the units that operated the Hammersmith & City services.

[13] In 1967, London Underground experimented with injection moulded plastic seats in the centre of CP Stock trailer 014082, in an attempt to address the then-emerging problem with vandalism.

The London Underground did not pursue the idea further, because passengers considered them to be uncomfortable, but vandal-resistant plastic seats became common on many metro systems, such as Algiers, Barcelona (TMB lines), New York City and Shanghai.