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.[1] The type 2 boiler had a tube surface of 1,669 square feet, formed by tubes 2+1⁄8 inches (54 mm) in diameter and 14 ft 3 in (4.34 m) long.[2] It was fitted with a 28-row superheater with 360 square feet (33 m2) of heating surface.[1] The superheater elements, 1+1⁄8 inches (29 mm) in diameter, were fitted into flue tubes 5+1⁄8 inches (130 mm) diameter.[2]
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.[1] 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.[2] The boiler was re-tubed with 180 smaller tubes 1+7⁄8 in (48 mm) diameter, and with superheater elements 1+1⁄4 in (32 mm) diameter.[2] The single blastpipe was replaced with a double blastpipe and chimney to provide adequate gas velocity in the smokebox.[1] This, combined with the increased rate of evaporation provided by the re-tubing, improved the performance of the boiler substantially.[2]
The performance of Stanier's Jubilee Class also suffered from incorrect boiler proportions.[2] In 1935, a design for an improved taper boiler was schemed out; this was 13 ft (4.0 m) long, with 198 tubes 1+3⁄4 inches (44 mm) in diameter.[2] In 1943 Stanier instructed Coleman, the chief designer at Derby, to rebuild two of the Jubilee class with boilers to this design.[2] This type of boiler was designated type 2A.[3] 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.[4]
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.[4] 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.