Mallard

Mallard is an A4 Pacific class steam locomotive built in the 1930s by the LNER and designed by Sir Nigel Gresley in England. It was designed as an express locomotive with awind-tunnel tested, aerodynamic body which allowed it reach speeds of over 100 mph. It was in service until 1963 when it was retired after a lifetime distance of almost 1.5 million miles. It was restored to working order in 1988 to celebrate its 50th anniversary, and is now part of the collection of the National Railway Museum in York, England. Gresley 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.