Extract from Wikipedia on the 3 cylinder version of what everyone knows as the 2 cylinder version which was the basis of the BR Standard 4MT

LMS 3-Cylindered Stanier 2-6-4T is a class of steam locomotive designed for work over the London, Tilbury and Southend Railway. All 2500-36 were built in 1934 at Derby Works. The third cylinder was provided to allow increased acceleration between the many stops on the L.T.&S.R. line. From 1935 the LMS switched to constructing a very similar, simpler, 2-cylindered design.

No. 2500 was the first of the 37 express passenger tank locomotives designed specifically for the London Tilbury and Southend line of the London Midland & Scottish railway (LMS) as they were too powerful for mixed-traffic work. It was built in 1934 at LMS Crewe works.

No. 2500 has survived and is on display at the National Railway Museum in York. It is the sole survivor of his class and is painted in LMS lined black livery.