inward swinging centre doors that were under the control of the gatemen. From 1915 the Bakerloo was extended to a junction with the London & North Western Railway (LNWR) at Queen's Park and because of the difficulties of World War I, cars were transferred from the Piccadilly line. From 1917 tube trains ran through to using Central London Railway motor cars that had been built for an uncompleted extension to Ealing. These returned in 1920 when the extension opened and the Watford Joint Stock, two thirds owned by the LNWR, arrived. Four and six car trains were formed, a motor car permitted in the middle of a tube train as there was an emergency passage through the control compartment. Access was by three single inwardly swinging doors on either side of the cars. Cars with sliding pneumatic doors were ordered in 1919 for the Piccadilly line. No gatesmen were needed and these trains were staffed by a driver and two guards. An open door was first indicated by a semaphore arm, but these were easily damaged, and by 1923 interlock switches proving the doors were closed were introduced. Standard Stock There were major extensions of the City & South London and the Hampstead lines in the 1920s. The tunnels of the City & South London Railway were rebuilt to have the same diameter of the other tubes, and extended north to a junction with the Hampstead line at Camden Town, and south to Morden. The Hampstead line was extended to Edgware and south to another junction with the City & South London at Kennington, this opening in 1926. New stock was required to run on the rebuilt City & South London and extensions so in 1922 six prototype cars were ordered from five manufacturers. With attention given to noise reduction, there were two pneumatic doors on each side of the trailers. From this development work a total of 1,460 cars of Standard Stock were built by six manufacturers in 18 batches between 1923 and 1934. Production cars were first ordered in 1923. The Hampstead line received the first of these later in 1923. Motor cars had a control compartment with a central gangway over the motored bogie and seated 30. The trailers seated 48 and the control trailers 44, with transverse and longitudinal seating. Initially trains had a crew of three, a driver and two guards. In 1928, once the