Gresley A4 Pacific Mallard is the holder of the world speed record for steam locomotives at 126 mph (approx 202 kph). The record was achieved on July 3, 1938 on the slight downwards grade of Stoke Bank, and the highest speed was recorded at milepost 90¼, between the towns of Little Blytham and Essendine.
Mallard was the perfect vehicle for such an endeavor; one of a class of streamlined locomotives designed for sustained 100mph+ running, it was one of a small number equipped with a double chimney and double Kylchap blastpipe, which made for improved draughting and better exhaust flow at speed. The A4's three-cylinder design made for better stability at speed, and the large 6'8" (2032mm) driving wheels meant that the maximum revolutions per minute was within tolerances.
Stoke Bank had a descending gradient of between 1:178 and 1:200; Mallard - with six coaches plus a dynamometer car in tow - topped Stoke Summit at 75mph and began to accelerate downhill. The speeds at the end of each mile from the summit were recorded at 87½, 96½, 104, 107, 111½, 116 and 119 mph; half-mile readings after that gave 120¾, 122½, 123, 124¼ and finally 125 m.p.h. The indicator diagrams on the dynamometer car traced a momentary maximum of 126mph.
Shortly following the attainment of this record speed, Mallard suffered an overheated inside big end bearing, and had to limp back to Doncaster for repair. Inaccuracies in the machining and setup of the Gresley-Holcroft derived motion (which derived the valve motion of the inside cylinder from those of the other two, avoiding a hard-to-maintain valve gear linkage between the frames) meant that the inside cylinder of the A4 did more work at high speed than the two outside cylinders; this overloading was mostly responsible for the failure.
Mallard's world record has never been officially exceeded for a steam locomotive, though German locomotives came very close. Many rumors and stories exist of higher speeds, but Mallard's is the only one with adequate documentation. Certainly many other steam locomotives were capable of such speeds; the LNER's long, straight, slightly downhill raceway of Stoke Bank played as much of a part in the record as the locomotive or crew.
Other locomotives that COULD have exceeded 126 mph include the New York Central's Niagara 4-8-4, the Pennsylvania Railroad's mighty S1 prototype (which is rumored to have exceeded 140mph) and T1, the Santa Fe's 2900 class 4-8-4s, and last but not least, the Milwaukee Road's A1 4-4-2 Atlantics and F7 4-6-4 Baltics. The Milwaukee Road had the fastest scheduled passenger trains in the world, with timetables requiring running in excess of 100mph; it's certainly known that they exceeded 120mph on a fairly frequent basis.
The story is that fear of lawsuits, and of a reputation for risk-taking through record runs, scared all American railroad companies away from official record attempts.
Thus, Mallard still holds the crown; a plaque affixed to each side of the locomotive commemorates the feat. When steam traction ended on British Railways, Mallard's survival was assured; it was made part of the National Collection at the National Railway Museum, York, England.
Thanks to help from www.gresley.org.uk and www.wandleys.demon.co.uk/mallard.htm | 
