The Great Western Railway Iron Duke Class 4-2-2 was a class of 7 ft 1⁄4 in (2,140 mm) broad gauge steam locomotives for express passenger train work.

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The prototype locomotive, Great Western, was built as a 2-2-2 locomotive in April 1846, but was soon converted to a 4-2-2 arrangement, with the leading wheels set rigidly within the sandwich framing, rather than in a separate bogie.^[1] The remainder of the class entered service between April 1847 and July

The Iron Duke locomotives were fast for their time, and were recorded reaching 78.2 mph (125.9 km/h).^[2] They were used to haul the Flying Dutchman express train, which was the fastest express train in the world for several decades. In 1852, the daily service from London Paddington to Exeter (194 miles or 312 km) was achieved with an average speed of 53 mph (85 km/h), with the flatter section between London and Swindon covered at an average speed of 59 mph (95 km/h).^[3]

From about 1865, the Iron Duke Class was known as the Alma Class.

In May to July 1870, three locomotives (Great Britain, Prometheus and Estaffete) were extensively rebuilt with new frames and boilers, but retaining their original names. Following these, further locomotives were built to similar specifications, entering service between August 1871 and July 1888. These new locomotives are generally referred to as the Rover class. Although these locomotives took the names of withdrawn locomotives of the original design, they were not rebuilt from them like the first three, but entirely new locomotives (though it is believed that Rover, Swallow and Balaklava may have included some parts from the earlier locomotives of those names).^[4]

Apart from the three conversions, the original locomotives were withdrawn between December 1870 and June 1884. Lord of the Isles (the last to be withdrawn) was initially preserved by the GWR at Swindon Works, but was scrapped in January 1906 owing to the pressure of space. The three conversions were withdrawn between September 1880 and October 1887, while the other locomotives to the later design were all withdrawn with the end of the GWR 7 ft 1⁄4 in (2,140 mm) broad gauge in May 1892 (except Hirondelle, which had been withdrawn in December 1890).

Many of the nameplates can be seen at the National Railway Museum and at the Museum of the Great Western Railway, while the driving wheels from Lord of the Isles can also be seen at Swindon.