[2] They were designed by the railway's Chief Mechanical Engineer, Charles Collett, for working the company's express passenger trains.

[4] The origins of this highly successful design date back to the Star Class of 1907 which introduced the basic 4-cylinder 4-6-0 layout with long-travel valves and Belpaire firebox that was to become characteristic of Great Western Railway (GWR) express passenger locomotives.

Charles Collett succeeded Churchward as Chief Mechanical Engineer of the GWR in 1922 and immediately set about meeting the need for a new locomotive design that would both supplement the Stars and replace them on the heaviest expresses.

Collett's solution was to take the basic layout of the Star with an extended frame, and add a newly designed No.8 boiler which was both larger and lighter.

The result was an increase in tractive effort to 31,625 lb, and a locomotive that looked attractive and well proportioned while remaining within the 20-ton axle limit.

Collett was sufficiently confident of the design to place an order with Swindon Works (Lot 224) for ten locomotives in 1923, although there was a four-month delay between the appearance of the first example in August 1923 and the second in December, to allow for the correction of any teething problems.

However, from 5013 Abergavenny Castle there was an alteration to the shape of the front-end casing over the inside cylinders, and from 5043 Earl of Mount Edgcumbe a shorter chimney was fitted.

4016 The Somerset Light Infantry (Prince Albert's), was similarly converted to a Castle although in this case, it retained its name and number.

The choice of 4082 as Windsor Castle proved fortuitous as this locomotive was used to haul the Royal Train when King George V and Queen Mary visited Swindon Works in 1924, and much publicity was gained when the king was invited to drive the engine back from the works to the station before the return journey, with the Queen and several high-ranking GWR officers also on the footplate.

The Great Western declared their engine to be more powerful than its bigger LNER rival, and in terms of tractive effort alone they were entitled to do so.

On the first morning Pendennis Castle was to work a 480-ton train from King's Cross to Doncaster, and LNER officials fully expected the smaller, lighter engine to encounter problems climbing Holloway Bank.

However, railway writer Cecil J. Allen records that the GWR locomotive made a faster start from King's Cross to Finsbury Park than any LNER Pacific he had recorded up to that time,[11] and over the trial Pendennis Castle kept well within the scheduled time and used less coal, considerably denting LNER pride.

For the LNER, Victor Wild was compared on the Cornish Riviera Express to 4074 Caldicot Castle and although it kept to time the longer wheelbase of the Pacific proved unsuited to the many curves on the route.

Again the GWR took the honours with Caldicot Castle burning less fuel and always ahead of time, this being illustrated on the last 2 days of the trial by gaining 15 minutes on the schedule in both directions.

The locomotive fulfilled the LMS requirements so well that the latter first requested the GWR to build a batch of Castles for use on the West Coast Main Line, and, failing that, a full set of construction drawings.

The application of shaped steel sheet in an attempt to smooth airflow has been described as a "bodge-up" and certainly lacked either the elegance of the A4 design, or the sense of power associated with the streamlined LMS Coronation Class introduced 2 years later.

[9] In 1946 Frederick Hawksworth, Collett's successor, introduced a higher degree of superheat to the Castle boiler with resulting increased economy in water consumption.

From 1956 the fitting of double chimneys to selected engines, combined with larger superheaters, further enhanced their capacity for sustained high-speed performance.

[21] The same locomotive was requested for the funeral of King George VI in February 1952; however, Windsor Castle was under repair at Swindon, so the number, name plate and commemorative plaques were swapped with No.

4079 was purchased by Sir William McAlpine and hauled a small number of railtours on the main line in its early preservation years before being sold to Hamersley Iron in the Pilbara region of Western Australia and exported in 1977.

[26] In 2000 it was donated by Rio Tinto to the Great Western Society and restored to operational condition at the Didcot Railway Centre in 2021.

The engine will only operate on heritage railways at first, but a full mechanical overhaul to mainline standards will be undertaken once enough money has been raised.