The steam locomotive that set the world steam-train speed record of 126 mph in 1938 was Mallard.
Mallard was an LNER Class A4 locomotive, numbered 4468, designed by Herbert Nigel Gresley for the London and North Eastern Railway. On 3 July 1938, it reached 126 mph (203 km/h) on a downhill section of the East Coast Main Line near Stoke Bank in Lincolnshire. The run took place while hauling a test train of railway coaches.
The record was achieved during the final years of British main-line steam power. Mallard was specially suited to high speed because its streamlined design reduced air resistance and its powerful Pacific wheel arrangement provided stability. The locomotive suffered an overheated big-end bearing after the run, so it was withdrawn for repairs. Mallard is preserved at the National Railway Museum in York. The record remains widely recognized as the fastest officially recorded speed by a steam locomotive, although claims involving unverified or experimental runs are sometimes confused with it.