LNER No. 4468 Mallard is a Class A4 4-6-2 Pacific steam locomotive designed under Sir Nigel Gresley and completed in 1938.
The A4 class used a streamlined casing developed for high-speed express passenger service.
Mallard was fitted with a double Kylchap exhaust system that improved steam flow and contributed to high-power operation at speed.
On July 3, 1938 the locomotive took part in a test run on the East Coast Main Line over Stoke Bank.
The train carried a dynamometer car that recorded the speed independently of the locomotive’s cab instruments.
During the descent Mallard reached a recorded peak of 126 mph.
That figure became the recognized world speed record for a steam locomotive and remains the record today.
The run was not mechanically effortless: the locomotive developed a hot big-end bearing afterward and required attention.
Mallard later returned to ordinary service and eventually entered the National Railway Museum collection.
For TransitSatellite, the 126 mph figure should be tied to the dynamometer evidence and the specific July 3, 1938 test. The page should not imply that 126 mph was a normal sustained operating speed for everyday A4 service.