LNER Gresley Classes A1 and A3
The London and North Eastern Railway Gresley Classes A1 and A3 were "Pacific" steam locomotives designed by Nigel Gresley for passenger work. They were initially intended for use on the Great Northern Railway, but became a standard design on the London and North Eastern Railway after the amalgamation of 1923. The change in class designation from A1 to A3 reflected the fitting to the same chassis of a higher pressure boiler with a greater superheating surface and a small reduction in cylinder diameter, leading to an increase in locomotive weight and power. Eventually all but one of the A1 locomotives were rebuilt in this way, with no. 4470 being completely rebuilt as a Class A1/1.
The names for the locomotives came from a variety of sources. The first, Great Northern, was named after its parent company. Others were given the names of high-ranking railway officials, but most were given the names of famous racehorses. One was named after the company's most famous long-distance passenger train, the Flying Scotsman.
Only one member of the class survives into preservation: 4472 Flying Scotsman'','' which is preserved in the National Railway Museum's National Collection.
Design and construction history
Class A1
The new Pacific locomotives were built at the Doncaster "Plant" in 1922 to the design of Nigel Gresley, who had become Chief mechanical engineer of the GNR in 1911. The intention was to produce an engine able to handle, without assistance, mainline express services that were reaching the limits of the capacity of the Ivatt large-boilered Atlantics.Gresley's initial Pacific project of 1915 was for an elongated version of the Ivatt Atlantic design with four cylinders. Finally realising that he was in a design impasse, he took as a model the new American Pennsylvania Railroad class K4 Pacific of 1914. This in turn had been updated from a series of prototypes scientifically developed in 1910 under Francis J. Cole, Alco's Chief Consulting Engineer at Schenectady and the Pennsylvania's K29 Alco prototype of 1911, also designed by Cole. Descriptions of those locomotives appeared in the British technical press at the time and gave Gresley the elements necessary to design a thoroughly up-to-date locomotive.
The first two GNR Pacifics, 1470 Great Northern and 1471 Sir Frederick Banbury were introduced in 1922. The Great Northern board ordered a further ten '1470-class' locomotives, which were under construction at Doncaster at the time of the formation of the LNER in 1923. This included the future sole surviving member of the class, 4472 Flying Scotsman, then nameless and numbered 1472.
In line with the philosophy behind Cole's Alco prototypes, the Gresley Pacifics were built to the maximum limits of the LNER loading gauge with a large boiler and wide firebox giving a large grate area. The firebox was set low and rested on the trailing carrying axle. However, unlike the Pennsylvania K4, the firebox was not of the flat-topped Belpaire variety, but a round-topped one that was in line with Great Northern tradition. Features in common with the American types were the downward profile towards the back of the firebox and the boiler tapering towards the front. Heat transfer and the flow of gases were helped by use of a combustion chamber extending forward from the firebox space into the boiler barrel, along with a boiler tube length limited to, features inherited from the K4 type but not present on the earlier Cole Prototypes. The boiler pressure was rated at.
The 1470-class Pacific was the third Great Northern locomotive type to incorporate Gresley's universal 3-cylinder layout. All three cylinders drove the middle coupled axle. The outside cranks were set at 120°, with the inside crank displaced by about 7 degrees to allow for the 1:8 inclination of the inside cylinder, this slight deviation from even spacing being a suggestion by Harold Holcroft of the SECR which enabled the outside cylinders to be perfectly horizontal. Gresley conjugated valve gear derived the motion of the inside valve spindle from the two outside valve spindles: this eliminated an inaccessible middle set of valve gear between the frames. A feature of the K4 that had soon been abandoned by the Pennsylvania Railroad was an unusual three-bar version of the Laird slide-bar. However, Gresley adopted this type of slide-bar for all his locomotives and it was later taken up by Bulleid for his Pacifics and by Riddles for the British Railways standard designs.
Adaptation to the LNER
The Great Northern Railway amalgamated with several other railways to create the LNER as a result of the 1923 Grouping. Gresley was appointed Chief Mechanical Engineer of the new company, which was the second largest of the "Big Four" railway companies in Britain. Realising the need for standardisation, Gresley adopted his GNR Pacific design as the standard express passenger locomotive for the LNER main line, designating it 'A1' within the LNER locomotive classification system. The choice was made after comparative trials with an equivalent North Eastern Railway Pacific, classified 'A2'. Between 1923 and 1925, 51 A1 locomotives were built; twenty by the North British Locomotive Company, and the remainder by Doncaster Works. However, Gresley's Pacifics had been designed to work within the bounds of the Great Northern Railway, meaning maximum distances of less than 200 miles. After the grouping, the locomotives were required to have a far greater operating range.Early improvements
In 1924, 4472 Flying Scotsman was displayed at the British Empire Exhibition at Wembley along with the first member of the Great Western Railway Castle Class, 4073 Caerphilly Castle.In the following months, the two railway companies ran comparative exchange trials between the two types from which the Great Western emerged triumphant with 4079 Pendennis Castle. The LNER learned valuable lessons from the trials which resulted in a series of modifications carried out from 1926 on 4477 Gay Crusader. Changes to the valve gear included increased lap and longer travel, in accordance with Great Western practice; this allowed fuller exploitation of the expansive properties of steam and reduced back pressure from the exhaust, transforming performance and economy. The economies in coal and water consumption achieved were such that the Pacifics could undertake long-distance non-stop runs that had previously been impossible. There followed a complete redesign of the valve gear, which was applied to 2555 Centenary in 1927, with the rest of the class being modified in due course. Locomotives with modified valve gear had a slightly raised running plate over the cylinders in order to give room for the longer combination lever necessary for the longer valve travel. Another modification was made in 1927 when 4480 Enterprise was fitted with a 220 psi boiler. This was closely followed by two other locomotives which also incorporated variations in the cylinder diameter and superheater size for comparative purposes. This led Gresley to make a departure from Churchward practice by increasing the number of large tubes containing superheating elements: the greater superheater surface area in contact with hot gas raised steam temperature..
Most of the locomotives built as class A1 had vacuum brakes for both locomotive and train. However, the LNER had inherited a substantial number of Westinghouse-braked coaches, mainly from the NER, and it was necessary to provide some locomotives that were able to work trains formed of these coaches. Accordingly, fifteen of the forty A1s ordered by the LNER in 1923 had both Westinghouse and vacuum equipment for the train brakes, and on these the locomotive brakes were also Westinghouse. In 1928, the LNER decided to standardise on the vacuum brake, and as the number of Westinghouse-braked coaches subsequently decreased, the need for Westinghouse-fitted locomotives also fell. The fifteen A1s with Westinghouse brakes were converted to vacuum brakes between 1933 and 1935.
Class A3
The outcome of the various experiments and modifications made to the A1s in the late 1920s was a new Class A3 "Super Pacific", the first example of which was 2743 Felstead. This locomotive appeared in August 1928 with a 220 psi boiler, 19-inch cylinders, increased superheat, long-travel valves, improved lubrication and modified weight distribution. Another new development was the changeover from right to left-hand drive, less convenient for a right-handed fireman, but more convenient for sighting signals, resulting in the modification of all earlier locomotives.Twenty-seven A3s were built from new, with little variation, except for a new type of boiler with a "banjo dome", an oval steam collector that was placed on top of the rear boiler ring. The first banjo dome was hidden beneath the casing of Cock o' the North of 1934; and was subsequently used in the A4 streamliners. The last nine A3 Pacifics were constructed with the device in 1935, and it became a standard fitting on all LNER large, wide-firebox boilers that were applied to new locomotives until 1949, except for a short period while Edward Thompson was CME. The banjo dome was one of the Gresley features he disliked. It was also applied to replacement boilers on the A3s.
Although all of the original Class A1 locomotives were eventually rebuilt to Class A3 specifications, it was a drawn-out process that lasted until 1949; 60068 Sir Visto was the last locomotive to be converted. The changeover to left-hand drive took longer, and continued into the 1950s.
Further experiments
Despite having settled on a new standard type, Gresley continued to experiment on individual locomotives, in one of which experiments ACFI feedwater heaters were installed in A1 2576 The White Knight and A3 2580 Shotover. However, the increase in efficiency was deemed insufficient and the apparatus was eventually removed. In 1935, 2544 Lemberg received Trofimoff piston valves of a design with automatically varying steam passages.A3s 2747 Coronach and 2751 Humorist were subjected to smoke deflection trials following an accident on the London, Midland and Scottish Railway due to poor visibility; this included the modification of the upper smokebox area surrounding the chimney. Originally the whole smokebox wrapper was retained in order to form an air duct, with the exit behind the chimney, but this was found ineffective. The next experiment with 2751 Humorist was to cut off the top part of the wrapper, but retaining the sloping plate that directed air flow upwards, and therefore lifting the smoke above the locomotive. The original chimney was replaced by a double stove-pipe variety, and miniature deflector plates were added on each side, angled to concentrate the air flow when the locomotive was on the move.
Smoke-lifting devices were not a priority with the normal single-chimney Pacifics. However, with its double chimney and subsequent fitting of a double Kylchap exhaust in 1937, Humorist continued to pose a problem in this regard and always had small wings on each side of the chimney. Finally, in the 1950s, it acquired Peppercorn-type deflector plates.