[2][1] For example, they utilised a variation of both boiler and trailing truck of the Merchant Navy class, while weight was kept within the margins laid down by the Light Pacifics, all of which were designed by Oliver Bulleid.

[8][9] The "Britannias" had 6 ft 2 in (1.88 m) driving wheels, a compromise for their mixed-traffic role[3] to allow sustained fast running with passenger trains, yet small enough to give sufficient tractive effort for freight haulage.

A self-cleaning smokebox enabled ash to be ejected through the chimney, reducing the workload of the engine cleaner at the end of a working day.

[3] The single chimney was unusual for a "Pacific" type of locomotive, but was chosen because the exhaust dimensions including the blastpipe were designed using work done at the Rugby test plant and by S.O.

[11] The design also featured raised running plates above the wheels, which allowed easy access to the inside of the frames for purposes of lubrication.

[16] The initial order was for 25 locomotives, but such was the demand for the Britannias on the Eastern Region that more were rushed through construction before the teething problems had been ironed out on the prototypes.

[18] Problems with the class were experienced immediately, with the first 25 locomotives being withdrawn in October 1951 after several complaints were received from crews regarding the driving wheels shifting on their axles.

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.

An unusual fault with the first engines of the class was fore-and-aft vibration, strong enough to prompt passengers to complain and to cause fire-irons stowed in a longitudinal compartment on top of the tender to work their way forward into the cab.

Members of the third batch (70045–70054) were equipped with another tender design, the BR1D, which had 9 tons of coal and 4,750 gallons of water, due to the fact that they were intended for use on longer runs in the north of the railway network.

The two locomotives, which looked radically different from the rest of the class, were allocated to Manchester (Longsight) and ran a series of brake trials on the London Midland main line during the mid-1950s.

Western Region based examples had hand/foot holds cut into the smoke deflectors replacing the original handrails after a major derailment, to improve forward visibility.

[28] For this reason, the Western Region locomotive depots at Old Oak Common TMD and Plymouth Laira declared that the class was surplus to requirements.

[13][27] However Cardiff Canton depot displayed its liking for the class (despite being part of the former GWR empire) and managed to obtain good results on South Wales passenger traffic.

[13] The London Midland Region also had favourable reports, but a marked consistency in losing time on the longer runs between Holyhead and Euston was recorded, although all complaints were down to the individual techniques of the operating crews.

[29] The Southern Region also had an allocation of seven in May 1953, when all Merchant Navy Class locomotives were temporarily withdrawn for inspection after 35020 "Bibby Line" sheared a crank axle on the central driving wheel.

[30] Repairs to the class were undertaken at Crewe, Swindon and Doncaster Works until the financial constraints of the British Railways Modernisation Plan in terms of expenditure on steam began to preclude the regular overhaul of locomotives.

The first locomotive to be withdrawn from service was number 70007 "Coeur-de-Lion" in 1965, and the entire class was gradually transferred to Carlisle Kingmoor and Glasgow Polmadie depots as steam was displaced by the dieselisation of British Railways.

[43] Towards the end of steam plain green livery was substituted, with the touching-up of existing paintwork being preferred to full aesthetic overhaul.

[45] Number 70000 was originally selected to represent the class in the embryonic form of the future National Railway Museum, but was ultimately rejected due to the locomotive's poor mechanical condition.

However, 70000 had been purchased privately from British Railways by the Britannia Locomotive Group, which ensured that the doyen of the class was to survive into the preservation era.

[48][49] Subsequently, utilised on mainline railtours, the locomotive was out of use in the late 1990s, requiring work to bring it back to steam; it was eventually sold to Pete Waterman and stored at Crewe.