However, due to acute steel shortages in Britain, the order was continually postponed until it was finally cancelled on the publication of the 1955 Modernisation Plan for the re-equipment of British Railways.

[2] However, trials in other areas of the British Railways network returned negative feedback, a common complaint being that difficulty in steaming the locomotive made it hard to adhere to timetables.

The whole standardisation programme was launched with the building of the 7MT Britannia design in 1951; in the event, the 5MT proposal was dropped in favour of an updated version of the highly successful Stanier mixed traffic 4-6-0.

[5] It was further appreciated that a Pacific of 6MT power could be built with a high enough route availability to fulfil all remaining requirements; this had been amply demonstrated by Oliver Bulleid, Chief Mechanical Engineer of the Southern Railway, who had developed a lighter version of his large 3-cylinder Merchant Navy Class in 1945.

As the general policy of the Railway Executive was to eliminate as far as possible the perceived complication of multi-cylinder locomotives, an equivalent 2-cylinder Pacific was produced by mounting a smaller and lighter boiler on the standard 7MT chassis.

[7] The arrangement consisted of a modified Standard Class 7 boiler, with smaller steel cylinders and other modifications to save weight and hence increase route availability.

[8] The wider firebox, designed for use with cheaper imported coal, was also utilised to spread its weight evenly over the axles, whilst the standard smokebox completed the boiler, which at 225 lbf/in2 was rated at a lower working pressure than that of the Britannias.

[9] The design was fitted with a standard set of two Walschaerts valve gear systems, and all members of the class were equipped with 4,200 gallon BR 1 tenders.

[8] The initial order was for 25 locomotives, but such was the immediacy of demand regarding a smaller version of the Britannias that a batch of 10 was rushed through construction before teething problems had been ironed out at the British Railways testing station at Rugby.

[13] Initially, the return cranks on the main driving wheels were of LNER block-type as seen on Arthur Peppercorn's A1s and A2s, but this was changed to the simpler LMS four-stud fitting.

[16] However, had the Modernisation Plan been delayed, and the correct amount of investment made for undertaking the relevant modifications, such as streamlining of the steam passages and increased diameter blastpipe in a double-chimney layout, the Clans would have been free-steaming workhorses worthy of complementing the Britannias.

[17] Without modification, they were still capable machines when handled properly, as various feats testifying this included regular ascents of Shap and Beattock with 14 carriages without the assistance of a banking locomotive.

[18] Other arduous duties that the class frequently undertook were the regular turns on the Settle to Carlisle route, which has some of the steepest gradients and harshest working conditions of any British mainline.

[19] As more crews got used to them, the class could be found far from home territory at destinations as diverse as Aberdeen, Inverness, Port Talbot, Newcastle upon Tyne, Bristol, and even London.

[21] In August 1958, number 72009 was tested on the Eastern Region, being based at Stratford MPD, though a preference for the Britannias meant that this sojourn was short-lived, lasting only a month.

[26] Prior to the publishing of the Modernisation Plan advocating the change-over to diesel traction, there was a proposal to construct a second batch of the Clan Class, which was accepted as Crewe Works Order Lot 242.

[27] Originally scheduled for 1952 with frames constructed for 72010 Hengist, acute steel shortages meant that the order was continually rescheduled until the publication of the British Railways Modernisation Plan finally halted the project.

[28] The livery of the Clans was a continuation of the standard British Railways green applied to express passenger locomotives after nationalisation, lined in orange and black.