The steam locomotive that set the world speed record in 1938 was Mallard.
LNER Class A4 No. 4468 Mallard reached 126 mph (202 km/h) on 3 July 1938 during a brake test on Stoke Bank, near Grantham in England. It hauled seven coaches, including a dynamometer car that recorded the train’s speed. The streamlined locomotive had been designed by Sir Nigel Gresley for the London and North Eastern Railway.
Mallard was only four months old when it made the run. Its middle-cylinder big-end bearing overheated afterward, so the locomotive was sent to Doncaster for repairs. That mechanical failure did not erase the achievement: Mallard remains the accepted record-holder for steam locomotives.
A common mix-up is treating “Sir Nigel Gresley” or “LNER Class A4” as the answer. Gresley was the designer, while the A4 was the locomotive class. Flying Scotsman is another famous steam locomotive, but it did not set this record. Some historians debate whether 125 or 126 mph is the most defensible measured peak, but the locomotive’s name is not seriously in doubt.