LMS 2 and 2A boilered 4-6-0 locomotives

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{{Use British English|date=January 2017}}

The London, Midland and Scottish Railway (LMS) 2 and 2A boilered 4-6-0 locomotives were express passenger 4-6-0 steam locomotives. In 1935, William Stanier, Chief Mechanical Engineer of the LMS, ordered the rebuilding of the unique experimental high pressure compound locomotive 6399 Fury. The Schmidt-Henschel boiler was replaced with a tapered boiler, with a drumhead smokebox, designated type 2.{{sfn|Cox|1970|p=16}} The type 2 boiler had a tube surface of 1,669 square feet, formed by tubes {{convert|2+1/8|in}} in diameter and {{cvt|14|ft|3|in}} long.{{sfn|Cox|1970|p=17}} It was fitted with a 28-row superheater with {{convert|360|sqft}} of heating surface.{{sfn|Cox|1970|p=16}} The superheater elements, {{convert|1+1/8|in}} in diameter, were fitted into flue tubes {{convert|5+1/8|in}} diameter.{{sfn|Cox|1970|p=17}}

On testing the boiler performed poorly, with heat transfer to the water being inadequate because the hot gases from the firebox passed too rapidly through the tubes to the smokebox.{{sfn|Cox|1970|p=16}} The rapid passage of gas was indicated by too high a temperature in the smokebox, and the choking of the smokebox by excessive char carried through from the firebox.{{sfn|Cox|1970|p=17}} The boiler was re-tubed with 180 smaller tubes {{cvt|1+7/8|in}} diameter, and with superheater elements {{cvt|1+1/4|in}} diameter.{{sfn|Cox|1970|p=17}} The single blastpipe was replaced with a double blastpipe and chimney to provide adequate gas velocity in the smokebox.{{sfn|Cox|1970|p=16}} This, combined with the increased rate of evaporation provided by the re-tubing, improved the performance of the boiler substantially.{{sfn|Cox|1970|p=17}}

The performance of Stanier's Jubilee Class also suffered from incorrect boiler proportions.{{sfn|Cox|1970|p=17}} In 1935, a design for an improved taper boiler was schemed out; this was {{cvt|13|ft}} long, with 198 tubes {{convert|1+3/4|in}} in diameter.{{sfn|Cox|1970|p=17}} In 1943 Stanier instructed Coleman, the chief designer at Derby, to rebuild two of the Jubilee class with boilers to this design.{{sfn|Cox|1970|p=17}} This type of boiler was designated type 2A.{{sfn|Cox|1970|p=31}} Combined with modifications to the steam ports and valve gear, the rebuilding produced locomotives that were more powerful and economical than either the original Jubilees or the Royal Scots.{{sfn|Cox|1970|p=18}}

As the boilers of the Royal Scots were due for replacement, and their built-up smokeboxes were proving difficult to keep airtight, the decision was taken to rebuild the class in batches.{{sfn|Cox|1970|p=18}} They were also fitted with type 2A boilers. Rebuilding started in 1943, and eventually all 70 members of the class were rebuilt.

From 1946, 18 of the 52-strong Patriot Class (which shared the same chassis as the Royal Scots) also received 2A boilers.