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.
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 in (2.337 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. 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.