Class of 12+1 British 4-6-2 locomotives
The London, Midland and Scottish Railway (LMS) Princess Royal Class is a class of express passenger 4-6-2 steam locomotive designed by William Stanier. Twelve examples were built at Crewe Works, between 1933 and 1935, for use on the West Coast Main Line. Two are preserved.
Overview.
The designer of the class, William Stanier, had previously been Works Manager of the Great Western Railway's depot at Swindon Works, and had been recruited with a brief to replace the LMS's miscellany of locomotives inherited from its constituent companies. He made extensive use of Great Western features in his designs.
To match the power and speed and especially the prestige of the London and North Eastern Railway's express Pacific locomotives, Stanier designed the Princess Royal class almost as soon as he was appointed to the LMS. When originally built, they were used to haul the famous "Royal Scot" train between London Euston and Glasgow Central.
Design.
The class was based on GWR 111 The Great Bear, a design produced in 1907 for the Great Western by George Jackson Churchward. The smokebox and cylinders were closely based on those of the GWR 6000 Class (also known as the King Class). The inside cylinders were abreast the leading bogied wheels and drove cranks on the leading coupled axle, the outside cylinders were abreast the rear bogie wheels (which made substantial cross bracing necessary to brace the cylinders and the locomotive frame together) and drove crank pins on the centre coupled axle. Each of the four cylinders had its own set of Walschaerts valve gear.
Other minor details, such as corks to close oil boxes, closely followed Great Western practice.
Construction.
A prototype batch of three locomotives was to be constructed in 1933. Two were constructed as drawn but the third set of frames was retained as the basis for an experimental turbine locomotive.
Turbomotive.
The third prototype was constructed with the aid of the Swedish Ljungstrom turbine company and known as the Turbomotive, although not named. It was numbered 6202, in sequence with the Princess Royals. Although 'generally similar' to the rest of the Princess Royals, and 'not all that much different', it used a larger 40 element superheater to give a higher steam temperature, more suitable for turbine use. This boiler was also domeless as would later be used for the second batch of the Princess Royals. The continuous exhaust of the turbine, rather than the sharper intermittent blast of the piston engine, also required changes to the draughting and the use of a double chimney. It entered service in June 1935 on the London–Liverpool service.
This "Turbomotive" was rebuilt in 1952 with conventional 'Coronation' cylinders and named "Princess Anne", but was soon destroyed in the Harrow and Wealdstone rail crash.
Later production.
A second batch of eleven locomotives was constructed later. The first two locomotives of the class to be produced had a firebox combustion volume too small for the grate area, and the subsequent locomotives had enlarged fireboxes. 
Naming.
Each locomotive was named after a princess, the official name for the class was chosen because Mary, Princess Royal was the Colonel-in-Chief of the Royal Scots. However, the locomotives were known to railwaymen as "Lizzies", after the second example of the class, named for Princess Elizabeth, who later became Queen Elizabeth II. Later examples of 4-6-2 express passenger locomotive built by the LMS were of the related but larger, Coronation Class.
Withdrawal.
The class was withdrawn in the early 1960s in line with British Railways' modernisation plan.
Preservation.
Two examples, 6201 "Princess Elizabeth" and 6203 "Princess Margaret Rose" are preserved and both have operated on the mainline in preservation. They were named after the two children of Prince Albert, Duke of York (later King George VI), and his wife, Elizabeth, Duchess of York (later Queen Elizabeth, and after the king's death, Queen Elizabeth the Queen Mother). Princess Elizabeth Alexandra Mary (later Queen Elizabeth II) was seven years old in 1933 when her namesake was built, and Princess Margaret Rose was nearly five in July 1935 when her namesake was completed. At the time, they were third and fourth in line to the throne. 'Princess Margaret Rose' is owned by The Princess Royal Class Locomotive Trust and is on static display at the West Shed Museum, Midland Railway-Butterley, Ripley, Derbyshire.
Note: Loco numbers in bold mean their current number.
References.
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