GWR 4073 Castle Class - Rhubarb Loop

The GWR 4073 Castle Class was introduced in 1923 to meet the Great Western Railway’s increasing demand for more powerful express passenger locomotives. Designed by Charles Collett, the Castle was a refined development of George Churchward’s successful Star Class. It combined a larger boiler with a lengthened frame and improved steam circuit.

These 4-6-0 locomotives became closely associated with the Western Region’s fastest services, regularly hauling named expresses such as the Cheltenham Flyer and the Cornish Riviera Express at speeds in excess of 90 mph. Elegant, fast, and well proportioned, the Castles were widely regarded as among the finest express locomotives of their era.

Design and Development

Collett was tasked with producing a locomotive that could outperform the Stars in both power and efficiency, while remaining within the GWR’s 20-ton axle load limit. His solution was the Swindon Standard No. 8 boiler, which was larger and more efficient than its predecessors, yet light enough to allow broad route availability.

To match the increased steam output, the Castle Class used four cylinders (two inside and two outside), each measuring 16 in × 26 in. Long-travel piston valves and a refined steam circuit contributed to improved performance. Boiler pressure was set at 225 lbf/in², giving a tractive effort of 31,625 lbf, which was nearly ten percent higher than that of the Star Class.

Collett also introduced cab side windows, an enlarged grate area, and more balanced weight distribution. The first ten engines, beginning with 4073 Caerphilly Castle, entered service in 1923 and immediately proved the effectiveness of the new design.

Production and Variants

A total of 171 Castles were built between 1923 and 1950. This included:

155 newly built engines constructed at Swindon Works
16 rebuilds from earlier Star Class locomotives, including The Great Bear

Earlier Castles were paired with 3,500 gallon tenders, but from 1926 onwards a 4,000 gallon version became standard. The design was subtly revised over time, with modifications to the front-end casing and the adoption of a shorter chimney from No. 5043 onwards.

Frederick Hawksworth, Collett’s successor, introduced additional improvements in the 1940s, including larger superheaters and the fitting of double chimneys to some members of the class. These changes led to improved high-speed performance and greater fuel efficiency.

Performance and Publicity

The Castles quickly earned a reputation for exceptional performance. In 1924, 4073 Caerphilly Castle was displayed at the British Empire Exhibition at Wembley, where it was positioned alongside the LNER’s Flying Scotsman. This led to a series of locomotive trials between the two companies.

One of the most famous achievements occurred in 1932, when 5006 Tregenna Castle covered the 77¼ miles from Swindon to Paddington in just 56 minutes. The average speed was over 81 mph, which at the time set a world record for steam traction.

The LMS also evaluated the class in 1926 by borrowing 5000 Launceston Castle. Although the GWR refused to build Castles for the LMS or share construction drawings, the experience influenced the LMS’s future locomotive development. William Stanier, who had been Works Manager at Swindon, was later appointed Chief Mechanical Engineer of the LMS.

Withdrawal and Preservation

Withdrawals began in 1950 and continued throughout the 1960s. The final member of the class, 7029 Clun Castle, was withdrawn in December 1965, shortly after hauling the last scheduled steam-hauled express from Paddington.

Eight Castles were preserved, including three obtained directly from British Railways. The remainder were rescued from Barry Scrapyard. Several have run on the main line in preservation, and two remain certified for such use today.

Preserved Examples

4073 Caerphilly Castle — static display at STEAM, Swindon
4079 Pendennis Castle — operational at Didcot Railway Centre
5029 Nunney Castle — under overhaul
5043 Earl of Mount Edgcumbe — operational, main line certified
5051 Earl Bathurst — static display at Didcot
5080 Defiant — under overhaul at Tyseley
7027 Thornbury Castle — stored, future use under debate
7029 Clun Castle — operational, main line certified

4037 The South Wales Borderers

5015 Kingswear Castle

82A Bristol Bath Rd 20 Mar 1958 then to 87E Landore 16th Jun 1960 then OOC 25th Jan 1961 and to 88L Cardiff East Dock 4th Oct 1962 withdrawn 25th Apr 1963.

Castle Class No. 5015 Kingswear Castle at Taunton shed (83B) on 16 May 1959. © John Haydon.

5069 Isambard Kingdom Brunel

One of Laira’s ‘Boat Engines’. Rebuilt in 1958 with a double chimney, with the final large pattern of oil pipe cover between the boiler cladding and smokebox, mechanical lubricator in final position ahead of the outside steam pipe, no supplementary oil reservoir for the lubricator, outside steam pipes appear to have been changed to the later curved type. One thing which does show up is that the inside cylinder cover below the smokebox has gained the slightly raised central section which appeared on some of the later locos.

https://www.fox-transfers.co.uk/5069-isambard-kingdom-brunel-80785

https://www.flickr.com/photos/34208816@N03/4079880533/