'A3' class 4-6-2

Designed by Chief Mechanical Engineer Nigel Gresley
Rebuilt from original 'A1' design 1927-1948

CYLINDERS (3) 19 ins x 26 ins (482.6mm x 660mm)
COUPLED WHEEL DIAMETER 80 ins (2,032mm)
TOTAL HEATING SURFACE
(including superheater) 3,442 square feet (319.77m²)
GRATE AREA 41 square feet (3.81m²)
BOILER PRESSURE 220 PSI (1,516.8kpa)
TRACTIVE EFFORT 32,909 lbs (14,927kg)
TOTAL WEIGHT 160 tons (162.56 tonnes)
LENGTH OVERALL 70 feet (21,336mm)

In 1924 that most celebrated of all steam locomotives, Nigel Gresley's 'A1' class 4472 'Flying Scotsman' was a major exhibit at the British Empire Exhibition in Wembley. Also on display 4-6-0 'Castle' class 4073 'Caerphilly Castle' about which the Great Western Railway boasted as the most powerful express passenger locomotive in Great Britain.

Amid claims and counter claims, both companies agreed to exchange one of each others engines for performance and fuel consumption comparisons. In April 1925 yet to be named 'A1' 4474 was dispatched to Paddington and 'Castle' 4079 'Pendennis Castle' came to Kings Cross. 4474 was matched against 4074 'Caldicot Castle' over the London to Plymouth route and 'Pendennis Castle' battled with 'A1' 2545 between London and Doncaster. The trials proved to be rather embarrassing for the LNER as the smaller GWR locomotives outperformed the bigger 'Pacifics' hands down while using less fuel to do so. Subsequent clandestine checks made by the LNER engineers on another 'Castle' revealed the design of the valve settings enabled very short 'cut offs' resulting in more efficient use of the expansive nature of superheated steam in the cylinders. The 'Castle' also applied a higher boiler pressure, 225 PSI as compared to the 'A1's 180 PSI. The lesson was well learnt and the next couple of years saw significant improvements in fuel consumption and performance figures as redesigned valve gear was fitted and new boilers pressed to 220 PSI with increased superheating capacity replaced the old ones as overhauls became due. The new 'Super Pacifics' were reclassified 'A3' as they became available and eventually most of the 52 'A1's were converted, the last being 2567 'Sir Visto' (renumbered 60048 after nationalisation) in 1948.

In March 1935 'A3' 2750 'Papyrus' hauled a six car train from Kings Cross to Newcastle and return at an average speed of 80 mph with a maximum of 108 mph attained down Stoke Bank. This performance coupled with the success of an earlier trial with 'A1' 4472 'Flying Scotsman' proved to be the inspiration for the introduction of Gresley's definitive masterpiece, the streamlined 'A4' 'Pacific'.