LNER A4 Class Locomotive Mallard No.4468 at NRM York

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London and North Eastern Railway locomotive numbered 4468 Mallard is a Class A4 4-6-2 Pacific steam locomotive built at Doncaster, England in 1938. The locomotive is 70 ft (21 m) long and weighs 165 tons, including the tender. It is painted LNER garter blue with red wheels and steel rims.

Mallard is the holder of the world speed record for steam locomotives at 126 mph (203 km/h). The record was achieved on 3 July 1938 on the slight downward grade of Stoke Bank south of Grantham on the East Coast Main Line, and the highest speed was recorded at milepost 90¼, between Little Bytham and Essendine. It broke the German (DRG Class 05) 002's 1936 record of 124.5 mph (200.4 km/h). The record attempt was carried out during the trials of a new quick acting brake (the Westinghouse "QSA" brake).

The A4 class was designed by Sir Nigel Gresley to power high-speed streamlined trains. The wind-tunnel-tested, aerodynamic body and high power allowed the class to reach speeds of over 100 miles per hour (160 km/h), although in everyday service it rarely attained this speed. Whilst in BR days regular steam-hauled rail services in the UK were officially limited to a 90 mph 'line speed', pre-war, the A4s had to run way above 90 mph just to keep schedule on trains such as the Silver Jubilee and Coronation, with the engines reaching the magic three figures on many occasions. Mallard covered almost one and a half million miles (2.4 million km) before it was retired in 1963.

It was restored to working order in the 1980s, but has not operated since, apart from hauling some specials between York and Scarborough in July 1986 and a couple of runs between York and Harrogate/Leeds around Easter 1987. Mallard is now part of the National Collection at the United Kingdom's National Railway Museum in York.