LNER Class A4

The LNER Class A4 is a class of streamlined 4-6-2 steam locomotives 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 record as the world's fastest steam locomotive. Thirty-five of the class were built to haul express passenger trains on the East Coast Main Line route from London King's Cross via York to Newcastle, and later via Newcastle to Edinburgh. They remained in service on the East Coast Main Line until the early 1960s when they were replaced by Deltic diesel locomotives; they themselves proving to be worthy successors to the A4s. 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.

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 Silver Jubilee of George V.

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 miles (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.

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 No. 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 trainspotters as "streaks".

The streamlining side skirts (side 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.