and then by the Metropolitan Railway's own standard gauge locomotives. All tank engines, these locomotives were classified by letters of the alphabet. Initially eighteen A Class (4-4-0) were ordered in 1864. By 1870 a total of forty-four had been built. In 1885, an improved version was ordered and twenty-two B Class were built. From 1891, more locomotives were needed for work on the line from Baker Street into the country. Four C Class (0-4-4) were received in 1891 and six D Class (2-4-0) in 1894. From 1896 to 1901 seven E Class (0-4-4) locomotives were built to replace the A class on this line. In 1901 the Met also received four F Class (0-6-2), a freight variant of the E Class. Not all these new locomotives were fitted with the condensing equipment needed to work south of Finchley Road. The need for more powerful engines meant in 1915 four G Class (0-6-4) arrived, named after people or places associated with the Metropolitan Railway. Eight capable H Class (4-4-4) were built in 1920 for express passenger services, replacing the C and D class locomotives. In 1925, six freight K Class (2-6-4) locomotives arrived. These were out of gauge south of Finchley Road. London Passenger Transport Board In November 1937, the later Metropolitan Railway G, H and K Class steam locomotives were transferred to the London and North Eastern Railway (LNER), who took over all freight workings and became responsible for hauling passenger trains with steam locomotives north of Rickmansworth. From the early 1940s, these were replaced by ex Great Central Railway locomotives, now classified LNER Class A5. These were replaced in 1948 by LNER L1s. Former LMS locomotives replaced the L1s ten years later, when the joint line was transferred to British Railways' London Midland Region. Steam working ended on passenger trains in 1961, after electrification and the introduction of A Stock electric multiple units. Electric locomotives The City and South London Railway tube line opened in 1890 with electric locomotives hauling carriages. Initially the locomotive could haul three carriages at an average ; the trains were air braked, their reservoirs topped up at Stockwell. Fourteen locomotives were initially built, soon supplemented by more. When the railway was extended in 1900 a further 30 locomotives to an improved design were built, and 10 of the first generation rebuilt. In 1923 the railway was closed for reconstruction and tunnel enlargement, and the