LNER Class A4 4468 Mallard is a 4-6-2 ("Pacific") steam locomotive built in 1938 for operation on the London and North Eastern Railway (LNER) at Doncaster Works to a design of Nigel Gresley. Its streamlined, wind tunnel tested design allowed it to haul long distance express passenger services at high speeds. On 3 July 1938, Mallard broke the world speed record for steam locomotives at 126 mph (203 km/h), which still stands.

While in British Railways days regular steam-hauled rail services in the UK were officially limited to a 90 mph 'line speed', before the war, the A4s had to run significantly above 90 mph just to keep schedule on trains such as the Silver Jubilee and The Coronation, with the engines reaching 100 mph on many occasions. Mallard covered almost one and a half million miles (2.4 million km) before it was retired in 1963.

The locomotive is 70 ft (21 m) long and weighs 165 long tons (168 tonnes, 369,600 lbs), including the tender. It is painted LNER garter blue with red wheels and steel rims.

Mallard is now part of the National Collection and preserved at the National Railway Museum in York.