[2][3] By 1918, it was apparent to the GWR chief mechanical engineer George Jackson Churchward that his Star Class 4-6-0 locomotives would soon be incapable of handling the heaviest West of England expresses without assistance.

He therefore proposed fitting the 6 ft (1.83 m) diameter boiler used on his 4700 Class 2-8-0 on to a 4-6-0 chassis, in 1919, to create a more powerful express locomotive, but was prevented from doing so by the weight restrictions on the GWR main line.

[5] The GWR's General Manager, Sir Felix Pole, was anxious for a new design that would once again enable the company to claim to run the most powerful locomotive.

This was coupled with the widely known (but as yet unpublished) findings of the Bridge Stress Committee, which gave engineers a better scientific understanding of the impact of hammer blow, and enabled the GWR Civil Engineer to agree to raise the maximum allowable axle-loading to 22.5 long tons (22,900 kg) for the new ‘Super Castle’ class.

[7] Although Collett was nominally responsible for the design of the class, the detailed work was undertaken by his Chief draughtsman Frederick Hawksworth.

A replacement was built (Lot 309) which may have incorporated some parts from the damaged locomotive; it took the same number and name, and was added to stock on 24 March 1936.

[11] Due to their size and weight, the King class was however restricted to the London-Taunton-Plymouth (via both Bristol and Westbury) and the London-Birmingham-Wolverhampton (via Bicester) main lines.

They were unable to serve in Cornwall, due to the weakness of the Royal Albert Bridge, and so when they were hauling the Cornish Riviera Limited, they had to be swapped for a 'Castle' or 'Hall' at Devonport.

6000 was shipped to North America in August 1927 to join in Baltimore & Ohio Centenary celebrations, where its sleek appearance and smooth performance impressed all who witnessed it.

The application of pressurised oil lubrication showed its advantages over the largely grease-lubricated American Locomotives, and was even incorporated into a later design for the B&O in 1928.

[10] The class proved to be capable and reliable when using the high-calorific South Wales steam coal, on which the GWR had always relied for its good locomotive performance.

Following successful testing the whole of the class was subsequently modified and, as a result, their final years in British Railways ownership saw the very best of their performance, particularly on the steep South Devon Banks at Dainton, Rattery, and Hemerdon.

There have been two serious accidents involving the class: Thirty-one locomotives were built at Swindon, although only 30 were in service simultaneously:[18][19] As a result of its previous 7 ft 1⁄4 in (2,140 mm) broad-gauge system, the GWR had the largest loading gauge of all the pre-nationalisation railways in the UK.

6018 King Henry VI was the subject of preservation by Sir Billy Butlin, but the plans never came to fruition and the locomotive was scrapped.

[23] Developments in high-speed rail from the 1970s mean that ballast depths have increased, resulting in a present decrease in UK pan-network loading gauge height.

This has recently started to be reversed with the introduction of pan-European loading gauge standards on some mainlines, mainly originating from ports.

Faced with a choice of either not operating their locomotives on the mainline or modifying to allow them to pass within the current restricted UK loading gauge, private societies choose to reduce the height of their locomotives by 4 inches (102 mm) by: reducing cab and chimney height; modifying some upper pipe work.