Gresley LNER A4 Pacific Mallard is the holder of the world speed record for steam locomotives at 126 mph (approx 202 km/h). The record was achieved on July 3, 1938 on the slight downwards grade of Stoke Bank south of Grantham on the East Coast Main Line, 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 100+ mph 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" (2032 mm) driving wheels meant that the maximum revolutions per minute was within the capabilities of the technology of the day.
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 75 mph 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 mph. The indicator diagrams on the dynamometer car traced a momentary maximum of 126 mph.
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.
It is notable that, unlike the world records for automobiles, there is no requirement for an average of two runs in both directions, and assistance from gradient or wind has always been acceptable in rail speed records.
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 140 mph in somewhat debatable accounts) 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 steam-powered passenger trains in the world, with timetables requiring running in excess of 100 mph; it's certainly known that they exceeded 120 mph on a fairly frequent basis.
The belief is that—as far as can be ascertained—fear of lawsuits, and of a reputation for risk-taking through record runs, scared all American railroad companies away from official record attempts in the 1930s and 1940s.
Thus, Mallard still holds the crown; a plaque affixed to each side of the locomotive commemorates the feat.
The Record
Controversy
Mallard was also the name of a band featuring ex-members of Captain Beefheart & the Magic Band.
A mallard is also a type of duck