The steam locomotive generally credited with the world speed record for steam traction is LNER Class A4 4468 Mallard.
On 3 July 1938, Mallard reached an officially recorded 126 mph, or about 202.6 km/h, while hauling a seven-coach train down Stoke Bank on England’s East Coast Main Line. The run formed part of a test of the locomotive’s quick-acting braking system. A dynamometer car recorded the speed, and Mallard’s crew included driver Joseph Duddington and fireman Thomas Bray.
Mallard was a streamlined 4-6-2 Pacific designed by Nigel Gresley and built at Doncaster Works. Its aerodynamic casing, large driving wheels and powerful three-cylinder arrangement suited high-speed express work. The locomotive had already entered service only four months before its record run.
The record has a technical footnote: the train was descending, and Mallard suffered an overheated middle-cylinder big-end bearing immediately afterward. Germany’s DRG Class 05 No. 002 had reached 200.4 km/h in 1936 on level track, so enthusiasts sometimes debate the fairest comparison. Nevertheless, Mallard remains the accepted official record holder.