The story of the Princess Coronation Class is one of split personalities and purposes and represents the difference in opinions and attitudes between design on the one hand, and perceived cost savings on the other. The original streamlined form of the class arose from tests by the London Midland Scottish Railway’s Research Department on Stanier’s development of the Princess Royal class locomotives. Stanier was said to be ambivalent about the economic value of streamlining, but was aware of the publicity value of such a design. When the opportunity to produce five conventional versions of the Coronation Pacific as a comparison to the streamlined version arose, Stanier was quoted as saying, “They can have their streamliners if they b***** want them, but we will also build them five proper ones".
The Coronation or Princess Coronation class was the culmination of Sir William Staniers' pacific design process, an enlarged and improved version of the 12 Princess Royal class pacific which Stanier had designed to handle the London to Glasgow expresses over the entire route, including the steeply graded ascent of the Shap and Beatock. Originating from 6201 Princess Elizabeth (named for the Princess Elizabeth, HRH The Queen) known to railwaymen as 'Lizzies' the Princess Coronation class were the 'Big Lizzies', though with many of the class carrying Duchess titles 'Duchesses' is also widely used.
There is no doubt that the Coronation was a powerful locomotive and possibly the most powerful British steam locomotives built. Calculations from train loads and timings suggest these engines were capable of delivering up to 3,300 horsepower and with 38 locomotives built between 1937 and 1948 further developments were incorporated in later engines.
The last two, 6256 and 6257 were ordered by Staniers' successor as chief mechanical engineer of the LMS company George Ivatt with the final modifications to the class. These two engines rode rolling bearings in hard wearing manganese steel guides, higher temperature superheating, revised rear frame design to include a cast steel trailing truck, rocking firebox grate and hopper ashpan. Several of these work and maintenance saving features would be seen again in the British Railways standard locomotive designs of the 1950s.
6256 was the last Coronation class locomotive completed under LMS ownership and was named in honour of her designer, Sir William A. Stanier F.R.S. by Sir William himself.