LNER 4468
From Railways
| Loco Weight | Tender Weight | Driving Wheel | Boiler Pressure | Cylinders | Tractive Effort |
|---|---|---|---|---|---|
| 102 tons 19 cwt | 60 tons 7 cwt, 64 tons 19 cwt (Corridor) | 6' 8" | 250lb/sq in SU | Valve Gear: Walschaerts (piston valves) Three 18.5" x 26" | 33,616 lb |
| 104.60 tonnes | 61.32 tonnes, 65.99 tonnes (Corridor) | 2.03m | 1723.69 kPa | 0.47m x 0.66m | 149.53 kN |
London and North Eastern Railway (LNER) A4 class 4468 Mallard is an LNER 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 a wind-tunnel tested, aerodynamic body which allowed it reach speeds of over 100 mph (160 km/h). It was in service until 1963 when it was retired after a lifetime distance of almost 1.5 million miles (2,400,000 km). It was restored to working order in 1988 to celebrate its 50th anniversary, but at time of writing is out of service without a current boiler certificate. Mallard is now part of the national collection at the National Railway Museum in York, England.
[edit] The Record
Mallard is the holder of the world speed record for steam locomotives at 126 mph (203 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 Bytham and Essendine.
Mallard was the perfect vehicle for such an endeavor; one of a class of streamlined locomotives designed for sustained 100+ mph (160 km/h) 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 ft 8 in (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 (121 km/h) and began to accelerate downhill. The speeds at the end of each mile (1.6 km) from the summit were recorded at 87½, 96½, 104, 107, 111½, 116 and 119 mph (141, 155, 167, 172, 179, 187 and 192 km/h); half-mile readings after that gave 120¾, 122½, 123, 124¼ and finally 125 mph (194, 197, 198, 200 and 201 km/h). The indicator diagrams on the dynamometer car traced a momentary maximum of 126 mph (203 km/h).
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.
