A modification to Sir Nigel Gresleyís original A1 design, the LNERís A3 locomotives were the stalwarts of the East Coast Mail Line Express services from the mid-1920s, right up until 1961, when they were replaced by English Electric Deltics.
Almost as soon as Gresley was appointed Chief Mechanical Engineer of the Great Northern Railway in October 1911, he started to think about the design of large express passenger locomotives. His first designs were put into practice in 1915 and although the First World War delayed development, in April 1922, 1470 Great Northern became the first A1 to enter service.
Following exchange trials with the Great Western Railway, a range of modifications were brought to the A1 Class, resulting in them being upgraded into Class A3 Super Pacifics. Between 1927 and 1947, all of the A1s were rebuilt as Class A3 locomotives, with the exception of N0. 4470 Great Northern, which was rebuilt by Thompson in 1945 as a Class A1/1.
Smoke lifting experiments during 1932-33 using No.2751 Humorist were unsuccessful, this led to the fitting of the Kylchap double blastpipe arrangement with a lipped double chimney, but with no smoke deflectors. It was found that the softer blast from the Kylchap arrangement required the use of smoke deflectors and so small wing deflectors, on either side of the chimney, were replaced in 1947 by larger Peppercorn style deflectors.
Development of the class continued after Nationalisation in 1948, the remaining right hand drive A3s being converted to left hand drive between 1952 and 1954, whilst from 1954 A4 boilers were fitted to some of the A3s. This arose, not to improve performance, but because A3 spares were in short supply, however only about half of the A3s were rebuilt in A4 boilers. These boilers were in an un-streamlined form, and had their pressure reduced to 220psi.
Between 1958 and 1959, those remaining A3s without the modified Kylchap arrangement and double chimney were modified, the soft blast problem being solved by using German style smoke deflectors. These small improvements to the A3s worked well, but British Railís modernisation programme was well advanced and the first A3 was withdrawn in January 1966 and all were scrapped, except for No. 4472 Flying Scotsman, which was withdrawn in January 1963 and sold into preservation.
No. 2561 Minoru was built at Doncaster in May 1925 and was rebuilt as an A3 Class in June 1944. From new, Minoru was paired with an original GNR type Tender with coal rails fitted, this being changed for other versions several times, before being reunited with the loco in February 1955. Minoru was one of the later Kylchap arrangement, double chimney modifications, being converted in February 1959, the smoke deflectors fitted in July 1961. She was one of the locomotives that received the 107 A4 boiler, this was fitted in September 1962, before Minoru was finally withdrawn from service in December 1964.