The Class A4 is a class of streamlined 4-6-2 steam locomotive designed by Nigel Gresley for the London and North Eastern Railway in 1935. Their streamlined design gave them high-speed capability as well as making them instantly recognisable, and one of the class, 4468 Mallard, holds the world record as the fastest steam locomotive. Thirty-five of the class were built to haul express passenger trains on the East Coast Main Line route from London Kings Cross via York to Newcastle, and later via Newcastle to Edinburgh, Scotland. They remained in service on the East Coast Main Line until the early 1960s when they were replaced by Deltic diesel locomotives. Several A4s saw out their remaining days until 1966 in Scotland, particularly on the Aberdeen – Glasgow express trains, for which they were used to improve the timing from 3.5 to 3 hours.
Overview
Gresley introduced the Class A4 locomotives in 1935 to haul streamlined Silver Jubilee trains between London King’s Cross and Newcastle. The service was named in celebration of the 25th year of King George V’s reign.
During a visit to Germany in 1933, Gresley had been inspired by high-speed streamlined Flying Hamburger diesel trains. The London and North Eastern Railway (LNER) had considered purchasing similar trains for use from London to Newcastle but the diesel units of the time did not have sufficient passenger carrying capacity and the capital investment in the new technology was prohibitive.
Gresley was sure that steam could do equally well with a decent fare-paying load behind the locomotive. Following trials in 1935 in which one of his A3 Pacifics No.2750 Papyrus recorded a new maximum of 108 mph (173.8 km/h) and completed the journey in under four hours, the LNER’s Chief General Manager Ralph Wedgwood authorised Gresley to produce a streamlined development of the A3. Initially four locomotives were built, all with the word ‘silver’ in their names. The first was 2509 Silver Link, followed by 2510 Quicksilver, 2511 Silver King and 2512 Silver Fox. During a press run to publicise the service, Silver Link twice achieved a speed of 112.5 mph (181.1 km/h), breaking the British speed record and sustained an average of 100 mph (160.9 km/h), over a distance of 43 mi (69.2 km).
Following the commercial success of the Silver Jubilee train, other streamlined services were introduced: The Coronation (London-Edinburgh, July 1937) and the West Riding Limited (Bradford & Leeds-London & return, November 1937) for which more A4s were built.
Design
The A4 Pacifics were designed for high-speed passenger services. The application of internal streamlining to the steam circuit, higher boiler pressure and the extension of the firebox to form a combustion chamber all contributed to a more efficient locomotive than the A3; consumption of coal and water were reduced. A further design improvement was fitting a Kylchap double-chimney, first on 4468 Mallard in March 1938. The double-chimney improved the capability of the locomotives further, and the last three locomotives of the class (4901 Capercaillie, 4902 Seagull and 4903 Peregrine) were fitted with the Kylchap exhaust from new and the rest of the class acquired it in the late 1950s.
The class was noted for its streamlined design, which not only improved its aerodynamics, increasing its speed capabilities, but also created an updraught to lift smoke away from the driver’s line of vision, a problem inherent in many steam locomotives particularly those operated with short cut off valve events, fitting smoke deflectors was an alternative solution. The distinctive design made it a particularly attractive subject for artists, photographers and film-makers. The A4 Class locomotives were known by train spotters as “streaks”.
The streamlining side skirts (valances) designed by Oliver Bulleid to aerofoil shape that were fitted to all the A4 locomotives, were removed during the Second World War to improve access to the valve gear for maintenance and were not replaced. This apart, the A4 was one of few streamlined steam locomotive designs in the world to retain its casing throughout its existence. Many similar designs, including the contemporary Coronation class, had their streamlining removed or cancelled to cut costs, simplify maintenance and increase driver visibility.
World Record
On 3 July 1938 4468 Mallard; the first of the class to enter service with the Kylchap exhaust, pulling six coaches and a dynamometer car, set a world speed record (indicated by the dynamometer) of 126 mph (202.8 km/h). Gresley never accepted it as the record-breaking maximum. He claimed this speed could only have been attained over a few yards, though he was comfortable that the German speed record of 124.5 mph (200.4 km/h) had been surpassed. Close analysis of the dynamometer roll (currently at the NRM) of the record run confirms that Mallard’s speed did in fact exceed that of the German BR 05 002. The Mallard record reached its maximum speed on a downhill run and failed technically in due course, whereas 05 002’s journey was on level grade and the engine did not yet seem to be at its limit. On the other hand, the German train was four coaches long (197 tons), but Mallard’s train had seven coaches (240 tons). One fact, often ignored when considering rival claims, is that Gresley and the LNER had just one serious attempt at the record, which was far from a perfect run with a 15 mph permanent way check just North of Grantham. Despite this a record was set. Gresley planned another attempt in September 1939, but was prevented by the outbreak of World War II. Prior to the record run on 3 July 1938, it was calculated that 130 mph (210 km/h) was possible, and Driver Duddington and LNER Inspector Sid Jenkins both said they might well have achieved this figure had they not had to slow for the Essendine junctions.
At the end of Mallard’s record run, the middle big end (part of the motion for the inside cylinder) ran hot (indicated by the bursting of a heat-sensitive “stink bomb” placed in the bearing for warning purposes), the bearing metal had melted and the locomotive had to stop at Peterborough rather than continue to London. Deficiencies in the alignment of the Gresley-Holcroft derived motion meant that the inside cylinder of the A4 did more work at high speed than the two outside cylinders – on at least one occasion this led to the middle big end wearing to such an extent that the increased piston travel knocked the ends off the middle cylinder – and this overloading was mostly responsible for the failure.
From Wikipedia
