roof instead of the clerestory roof on the earlier designs. In 1920 the District took delivery of new cars, incompatible with the existing fleet, with three hand-operated double sliding doors on each side. In 1923, fifty motor cars were ordered to allow some of the original 1904–05 cars to be scrapped. In 1926, the District Railway classified its rolling stock using letters of the alphabet, the original prototype being A Stock, the production cars B Stock, and so on. In that year, two stock pools were created. Mainline trains were formed from 101 new motor cars supplemented by motor cars rebuilt from the steel-bodied cars originally constructed in 1910–14 and 1923, and trailers modified from the original wooden-bodied cars. A small pool of unmodified 'local stock' worked the shuttles from Acton Town to South Acton, South Harrow and Hounslow. Metropolitan Railway The first order for electric multiple units was placed with Metropolitan Amalgamated in 1902 for 50 trailers and 20 motor cars with Westinghouse equipment, which ran as 6-car trains. First- and third-class accommodation was provided in open saloons, second class being withdrawn from the Met. Access was at the ends via open lattice gates and the units were modified so that they could run off-peak as 3-car units. For the joint Hammersmith & City line service, the Met and the Great Western Railway purchased twenty 6-car trains with Thomson-Houston equipment. In 1904, a further order was placed by the Met for 36 motor cars and 62 trailers with an option for another 20 motor cars and 40 trailers. Problems with the Westinghouse equipment led to Thomson-Houston equipment being specified when the option was taken up and more powerful motors being fitted. Before 1918, the motor cars with the more powerful engines were used on circle services with three trailers. The open lattice gates were seen as a problem when working above ground and all of the cars had been modified to replace the gates with vestibules by 1907. Having access only through the two end doors became a problem on busy circle services and centre sliding doors were fitted from 1911. From 1906, some of the Ashbury bogie stock was converted into multiple units by fitting cabs, control equipment and motors. In 1910, two motor cars were modified with driving cabs at both ends. They started work on the Uxbridge-South Harrow shuttle service, before being transferred to the Addison