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 tons, 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.

A steam locomotive is a rail vehicle that provides the force to move itself and other vehicles by means of the expansion of steam.: 80  It is fuelled by burning combustible material (usually coal, oil or, rarely, wood) to heat water in the locomotive's boiler to the point when it becomes gaseous and its volume increases 1,700 times. Functionally, it is a steam engine on wheels.

In most locomotives, the steam is admitted alternately to each end of its cylinders, in which pistons are mechanically connected to the locomotive's main wheels. Fuel and water supplies are usually carried with the locomotive, either on the locomotive itself or in a tender coupled to it. Variations in this general design include electrically-powered boilers, turbines in place of pistons, and using steam generated externally.

Steam locomotives were first developed in the United Kingdom during the early 19th century and used for railway transport until the middle of the 20th century. Richard Trevithick built the first steam locomotive known to have hauled a load over a distance at Pen-y-darren in 1804, although he produced an earlier locomotive for trial at Coalbrookdale in 1802. Salamanca, built in 1812 by Matthew Murray for the Middleton Railway, was the first commercially successful steam locomotive. Locomotion No. 1, built by George Stephenson and his son Robert’s company Robert Stephenson and Company, was the first steam locomotive to haul passengers on a public railway, the Stockton and Darlington Railway, in 1825. Rapid development ensued; in 1830 George Stephenson opened the first public inter-city railway, the Liverpool and Manchester Railway, after the success of Rocket at the 1829 Rainhill Trials had proved that steam locomotives could perform such duties. Robert Stephenson and Company was the pre-eminent builder of steam locomotives in the first decades of steam for railways in the United Kingdom, the United States, and much of Europe.Towards the end of the steam era, a longstanding British emphasis on speed culminated in a record, still unbroken, of 126 miles per hour (203 kilometres per hour) by LNER Class A4 4468 Mallard. In the United States, larger loading gauges allowed the development of very large, heavy locomotives such as the Union Pacific Big Boy, which weighed 540 long tons (550 t; 600 short tons) and had a tractive effort of 135,375 pounds-force (602,180 newtons).From the early 1900s, steam locomotives were gradually superseded by electric and diesel locomotives, with railways fully converting to electric and diesel power beginning in the late 1930s. The majority of steam locomotives were retired from regular service by the 1980s, although several continue to run on tourist and heritage lines.