The Dean Single, 3031 Class, or Achilles Class was a type of steam locomotive built by the British Great Western Railway between 1891 and 1899. They were designed by William Dean for passenger work. The first 30 members of the class were built as 2-2-2s of the 3001 Class.

Achilles Class
Dean Single No. 3050 Royal Sovereign in photographic grey livery.
Type and origin Power type Steam Designer William Dean Builder GWR Swindon Works Order number Lots 94, 95, 110 Serial number 1391–1420, 1612–1631 Build date 1894–1899 Total produced 50 (new) Number rebuilt 30 (from 3001 class)
Specifications Configuration: ​  • Whyte 4-2-2  • UIC 2A1 n2 Gauge 4 ft 8+1⁄2 in (1,435 mm) standard gauge Leading dia. 4 ft 1+1⁄2 in (1.257 m) Driver dia. 7 ft 8+1⁄2 in (2.350 m) Trailing dia. 4 ft 7+1⁄2 in (1.410 m) Wheelbase 18 ft 8+1⁄2 in (5.702 m) Cylinder size 19 in × 24 in (483 mm × 610 mm)
Career Disposition All scrapped, one non-working replica was built in the early 1980s

The first eight members of the class (numbers 3021-3028, built April–August 1891) were built as convertible 7 ft 1⁄4 in (2,140 mm) broad gauge 2-2-2 locomotives, being converted to standard gauge in mid-1892, at the end of broad gauge running on the Great Western Railway. A further 22 were built in late 1891 and early 1892, this time as standard gauge engines.

Although the 4 ft 8+1⁄2 in (1,435 mm) 3001 class were fitted with larger boilers than earlier GWR 2-2-2 classes, the diameter of the boiler was constrained by its position between the 7 ft 8+1⁄2 in (2.350 m) driving wheels. Thus boiler capacity could only be increased by making the boiler longer, not wider, bringing the smokebox and cylinders in front of the leading axle.^[1] The extra weight of the larger boilers was borne by the leading wheels, making the locomotives unstable, particularly at speed. On 16 September 1893 No. 3021 Wigmore Castle, hauling an express train, was derailed in Box Tunnel when the front axle broke. The cause of the accident was thought to be excessive weight being carried on the front axle, so it was decided to replace the leading pair of wheels in the 3001 class with a bogie.^[2][3]

In the 3001 class the steam chest was located underneath the cylinders, and contained two slide valves. The inverted placement of the valves allowed them to drop away from the face of the steam ports when steam was shut off, thus reducing wear.^[4] The steam chest and valves lay above the front carrying axle, and there was sufficient clearance to allow the steam chest cover to be removed over the axle for maintenance.

Replacing the axle with a bogie of conventional design would have obstructed access to the port faces. Dean instead used a suspension bogie, in which the weight of the locomotive was transferred upwards to the bogie by four bolts mounted on the inside frames. The centre pin of the bogie rotated in a spring-centred block mounted beneath the steam chest on cross beams. This setup gave sufficient clearance so that, when the bolts were undone, the front end of the locomotive raised, and the bogie was run out from underneath, the steam chest cover could be removed without hindrance.^[2]

No. 3021 was rebuilt as a 4-2-2 in March 1894. Between June and December 1894 the 28 remaining locomotives of the 3001 class were rebuilt.^[5] The first of a further 50 new bogie singles was also built in March 1894, the last of the class being outshopped in March 1899.^[6] These new locomotives differed from the rebuilds in having their cylinder diameter reduced from 20 to 19 inches (508 to 483 mm), and the springs for the trailing wheels located above the footplate and outside the cab, necessitating a reduced width for the latter.^[7] The rebuilds subsequently had their cylinders lined down to 19 inches (483 mm).^[8] The entire class, as they required it, had their driving wheels fitted with thicker tyres from 1898 onwards, increasing the wheel diameter by one-half inch (12.7 mm) to 7 ft 8+1⁄2 in (2.350 m).^[8]

In 1900, George Jackson Churchward replaced the boiler on number 3027 Worcester with a parallel Standard 2 boiler. Twelve further engines were similarly converted in 1905 and 1906.

Despite the locomotives' speed, the 4-2-2 design was soon found to be outdated and unsuitable for more modern operation. A proposal to improve their performance by fitting them with long-travel valves was found to be impracticable; the existing valves were directly driven from eccentrics mounted on the driving axle, and there was insufficient clearance to fit larger eccentrics.^[9] Churchward considered rebuilding the class as Armstrong Class 4-4-0s with 7 ft 2 in (2.184 m) coupled wheels. The cylinder centre line would then be 3.5 in (89 mm) above the driving centre, due to the 7 in (178 mm) difference in driving wheel diameter. This scheme was not carried through because the connecting rods would not clear the lower slide bar, and the valve gear would be out of alignment. An alternative proposal to drop the locomotive 3.5 in (89 mm), and raise the buffer beam and dragbox, was also rejected on the grounds of cost. The class were gradually withdrawn between 1908 and 1915, with the last survivor, no. 3074 Princess Helena, being withdrawn in December 1916.^[9] All of the originals were scrapped, but a non-working replica was built in the early 1980s.^[10][11]