| Power Classification | Unclassified |
| Introduced | |
| Designer | Gooch |
| Company | GWR |
| Weight | 24t 4cwt |
| Driving Wheels | 8ft 0ins |
| Boiler Pressure | 140psi |
| Cylinders | Inside – 18 in x 24in |
| Tractive Effort | 8,262lbf |
| Valve Gear | gooch |
Isambard Kingdom Brunel built a broad gauge (7ft ¼) line between Paddington and Bristol for high speed running with gentle gradients and minimum curvature. Gooch was the locomotive builder who was aged only 20 when Brunel recruited him in 1837 as Locomotive Superintendent of the GWR. Gooch had previous experience in the employment of Robert Stephenson & Co.
The prototype locomotive and the first engine to be built at Swindon, Great Western, was built as a 2-2-2 locomotive in April 1846 and June of that year it achieved a speed of 74½mph. It was designed to show how the 2-2-2 express engines could be improved; its 8-foot-diameter driving wheels were a foot larger than those of the successful Fire Fly class but It broke its leading axle after a short while in service and was subsequently converted to a 4-2-2 as Gooch considered the problem was the uneven weight distribution. It was rebuilt with the leading wheels set rigidly within the sandwich framing, rather than in a separate bogie, becoming part of the Iron Duke class.
The remainder of the class entered service between April 1847 and July 1855.
These engines originally burnt coke but later coal was used. The original water capacity of the tender was increased from the original 1760 gallons to 1880 gallons but later tenders capable of holding 2,700 gallons were attached to the locomotives in order to allow them to run non-stop between Paddington and Swindon.
Locomotives of the Iron Duke class were extremely fast and had an estimated top speed of about 80 mph. In May 1848 a member of the Iron Duke class named Great Britain attained a speed of 78.2mph at Wootton Bassett, eight miles west of Swindon. They were used to haul the Flying Dutchman express train which, for several decades, was the world’s fastest train. In 1852 the daily service from Paddington Station to Exeter (194 miles) was achieved with an average speed of 53 mph; with the flatter section between London and Swindon covered at an average speed of 59 mph.
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).
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 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 broad gauge in May 1892 (except Hirondelle, which had been withdrawn in December 1890).
In 1985 a working replica of Iron Duke was constructed using parts from two Hunslet Austerity tanks (LNER class J94) as part of the Great Western 150 celebrations. The engine was built by Resco (Railways) Ltd at Woolwich and the tender by BR at Cathay Works in Cardiff. The replica was commissioned by the NRM in anticipation of the 150th anniversary of the incorporation of the GWR in 1985.
The boiler certificate has expired so it cannot currently be steamed.
It part of the National Railway Collection but is currently on long-term loan to the Didcot Railway Centre, which has a section of working broad gauge track.
| Home Base | Current Status | Owner |
| Didcot Railway Centre | On static display | National Railway Museum
NRM Object Number{1985-1989}
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Preserved British Steam Locomotives 