LNER 4-6-2 Locomotives in Great_Britain

Class Details by Steve Llanso of Sweat House Media

Class A3 (Locobase 3072)

Designed by Nigel Gresley. This was the result when the A1 (Locobase 1065) showed signs of excessive coal consumption. A redesign of the valve gear, a substantial increase in superheater area, and reduction of cylinder diameter to 19" seem to have done the trick. This class was a mixture of rebuilt A1s (52) and new engines. The latter -- 27 in all -- came in batches starting in August 1928 and ending in December 1935. One difference between the two classes was that the former was set up for right-hand drive, the latter for left-hand drive.

[] (5 Sept 2003) for boiler data.

Richard HN Hardy (see Locobase 2314 for a discussion of this photographer and his extensive collection of LNER images; see [], accessed 8 May 2006) offered a terse comparison of the relative amounts of reserve in the A3 vs the A1: "Those old A1s could time the heavy trains with their big cylinders even when down to 140 psi or so whilst an A3 or, even more, and A4 would be struggling if short of steam."

Class A4 (Locobase 1066)

Data from Richard Marsden in http://www.lner.info/locos/A/a4.html, consulted March 2002. See also Edward Cecil Poultney, British Express Locomotive Development (London: George Allen and Unwin, 1952), p. 58-59; G Freeman Allen, The Great Railroad Photographs of Eric Treacy ; and Jamie Duggan, "LNER A4 Class steam locomotives - Class Information", archived 30 September 2018 on the Rail Advent site at https://www.railadvent.co.uk/2018/09/lner-a4-class-steam-locomotives-class-information.html, last accessed 11 July 2019.

Designed by Nigel Gresley. Delivered with a distinctive streamlined casing, these engines quickly proved to be extremely fast and free-running. Marsden comments that the class was a hit from the start. In 1,952 Silver Jubilee runs before World War II, only 10 saw any mechanical problem offered by the A4. "Drivers and firemen were happy, too," says Marsden. "They found the A4s easily took the loads required of them. They also found the footplate very smooth."

Richard HN Hardy (see Locobase 2314 for a discussion of this photographer and his extensive collection of LNER images; see [], accessed 8 May 2006) offered a terse comparison of the relative amounts of reserve in the A4 vs the A1: "Those old A1s could time the heavy trains with their big cylinders even when down to 140 psi or so whilst an A3 or, even more, and A4 would be struggling if short of steam."

The Mallard set a steam-powered record of 126 mph in 1938. Perhaps two reasons for the class's free-steaming was a superheat ratio that was comparatively generous for British locomotive, internal streamlining of the steam passages, and 9" (229 mm) piston valves with steam laps of 1 5/8" (41 mm) and travel of 5 3/4" (146 mm). The Mallard in particular was fitted with a Kylchap double stack (chimney) and blastpipel.

The Gresley conjugated gear that ran the inside valve operated well, if it was well-maintained. But Duggan told of a continuing problem in the design: "The problem was caused by Gresley's Conjugated gearing, which at high speeds caused the middle cylinder to work harder than the outside cylinders. The strain on this cylinder would eventually cause it to overheat and the bearing to melt. [This happened to the Mallard after her 126 mph timing was confirmed and ended her run that day.] This resulted in the engine being withdrawn from traffic while repair work was carried out."

Duggan added that R. J. Cook worked out a remedy once he became Mechanical and Electrical Engineer for the Eastern Region of BR. "Originally from the Great Western Railway / Western Region, he brought over some practices on his move. Notably, he used the GWR type of bearing combined with tighter tolerances when reassembling the A4s. "

Altogether, 35 engines of this class were completed and ran into the late 1950s. G. Freeman Allen (writing in Great Railway Photographs of Eric Treacy, 1987) comments that the fitting of Klychap double blastpipes and chimneys (sic) "which so strikingly rejuvenated them" was "wretchedly belated," occurring only in the late 1950s.

See http://www.gresley.org.uk/snghome.htm for details on the rebuilding of the Sir Nigel Gresley.

As with many express passenger engines, once the A4s were supplanted by the diesel Class 55 Deltics, they were ill-suited for most other service. Withdrawals began in the early 1960s, but six of the class continued to haul trains in Scotland until 1966.

Class A5 (Locobase 3765)

Tufnell (1986) says this batch was a scaled-down A5 design with smaller boilers to fit the LNER's North Eastern lines' loading gauge. See the earlier locomotives at Locobase 2323.

. Data from Bryan Attewell ([])'s Steam locomotive simulator (April 2000 edition), confirmed and augmented by Richard Marsden's LNER site -- http://www.lner.info/locos/locos.shtml (Jan 2004) The data show a design consistent with British practice -- a hard-worked boiler, low superheat, generous firebox area, and a modest EHS to cylinder volume ratio.

As with the older 9N/A5, these locomotives provided solid service right through the 1950s. As newer designs replaced them in the mid-1950s, A5s retired, but the last didn't go out of service until 1960.

Class A8 (Locobase 5911)

Data from Richard Marsden, "The Raven/Gresley A8 Pacific Tank Locomotivess" from his LNER site -- https://www.lner.info/locos/A/a8.php (first accessed January 2004, last accessed 13 June 2020).

Marsden says that when the Class H 4-4-4Ts were compared to the A5 4-6-2Ts side-by-side, the advantage of a third adhesive axle became apparent. Tests with 2162 after its conversion to the Pacific tank arrangement in 1931 confirmed the improvement and all of the H1 4-4-4Ts were converted from 1933 to 1936.

Marsden comments thats the rebuilt A8s "could easily work the heavy suburban traffic and long-distance coastal trains on which they were put to work." Locobase considers the relatively small amount of evaporative heating surface area and wonders if the high stroke-to-bore ratio allowed every possible BTU to be wrung from the superheated steam entering the cylinders through 7 1/2" (191 mm) piston valves.

OS Nock reported (Locomotives of the NER, 1954), that the A8 conversion produced "among the smoothest riding engines I have ever travelled on.".

Once diesel railcars were introduced in the 1950s, the A8s' days were numbered and they were withdrawn between 1957 and 1960.

Principal Dimensions by Steve Llanso of Sweat House Media
Locobase ID3072 1066 3765 5911
CountryGreat BritainGreat BritainGreat BritainGreat Britain
Number in Class27351345
Road Numbers
Number Built273513
BuilderLNERLNERHawthorn LeslieGateshead
Valve GearWalschaertWalschaertStephenson
Locomotive Length and Weight
Driver Wheelbase (ft / m)14.50 / 4.42
Engine Wheelbase (ft / m)35.75 / 10.9032.75 / 9.9833.25 / 10.13
Ratio of driving wheelbase to overall engine wheebase 0.41
Overall Wheelbase (engine & tender) (ft / m)60.89 / 18.5671 / 21.64
Axle Loading (Maximum Weight per Axle) (lbs / kg)49,392 / 22,40449,280 / 22,35340,320 / 18,28939,872 / 18,086
Weight on Drivers (lbs / kg)148,288 / 67,262148,000 / 67,132120,960 / 54,867
Tender Loaded Weight (lbs / kg)143,696 / 65,179
Total Engine and Tender Weight (lbs / kg)374,416 / 169,832192,416194,656
Tender Water Capacity (gals / ML)5000 / 18.945000 / 18.942280 / 8.64
Tender Fuel Capacity (oil/coal) (gals/tons / Liters/MT)8 / 78 / 7 4.70 / 4
Minimum weight of rail (calculated) (lb/yd / kg/m)82 / 4182 / 4167 / 33.50
Geometry Relating to Tractive Effort
Driver Diameter (in / mm)80 / 203280 / 203267 / 170269 / 1753
Boiler Pressure (psi / kPa)220 / 15.20250 / 17.20180 / 12.40175 / 12.10
High Pressure Cylinders (dia x stroke) (in / mm)19" x 26" / 483x660 (3)18.5" x 26" / 470x660 (3)20" x 26" / 508x66016.5" x 26" / 419x660 (3)
Tractive Effort (lbs / kg)32,910 / 14927.7435,455 / 16082.1423,749 / 10772.3822,890 / 10382.74
Factor of Adhesion (Weight on Drivers/Tractive Effort) 4.51 4.17 5.09
Heating Ability
Tubes (number - dia) (in / mm)121 - 2.25" / 57121 - 2.25" / 57134 - 1.875" / 48131 - 1.75" / 44
Flues (number - dia) (in / mm)43 - 5.5" / 14043 - 5.25" / 13322 - 5.25" / 13318 - 5.25" / 133
Flue/Tube length (ft / m)19 / 5.7917.98 / 5.4811.75 / 3.5811.33 / 3.45
Firebox Area (sq ft / m2)215 / 19.98231.20 / 21.49141.30 / 13.13124 / 11.52
Grate Area (sq ft / m2)41.25 / 3.8341.20 / 3.8321.20 / 1.9723 / 2.14
Evaporative Heating Surface (sq ft / m2)2692 / 250.192576 / 239.411280 / 118.961085 / 100.80
Superheating Surface (sq ft / m2)636 / 59.11749 / 69.61178 / 16.54191 / 17.74
Combined Heating Surface (sq ft / m2)3328 / 309.303325 / 309.021458 / 135.501276 / 118.54
Evaporative Heating Surface/Cylinder Volume210.34212.31135.39112.41
Computations Relating to Power Output (More Information)
Robert LeMassena's Power Computation907510,30038164025
Same as above plus superheater percentage10,79912,66942744629
Same as above but substitute firebox area for grate area56,28771,09428,48624,955
Power L121,75328,77276307139
Power MT970.221285.77417.19

All material Copyright © SteamLocomotive.com
Wes Barris