The LMS Royal Scot Class is a class of 4-6-0 express passenger locomotive introduced in 1927. 70 members of this class were rebuilt by the LMS and its successor BR from LMS Royal Scot Class engines by the replacement of their life expired parallel boilers with a type 2A boiler over the period 1943–1955.

The rebuilds were quite substantial, requiring new boiler, frames and cylinders, but in most cases the original frame stretchers, wheels, cab and fittings were retained. The usual procedure was that as each locomotive arrived for rebuilding, it was stripped, and the identity transferred to a fresh frameset prepared using the parts recovered from the locomotive that had previously been rebuilt. The new frames were slightly shorter than the originals. Thus, most rebuilt examples retained their own cab, wheels etc., but most of the frame stretchers, and other integral parts of the frame were from the previously rebuilt loco.

All were withdrawn between 1962 and 1965 in accordance with the 1955 Modernisation Plan. No original Royal Scots in 'as built' condition survive, as all were rebuilt by 1955.Of the 70 engines to be rebuilt only 2 members of the class have survived into preservation.