They were designed by William Stanier based on the earlier LMS Fowler 2-6-2T.

A taper-boiler versions of Henry Fowler's 1930 design, the general dimensions were the same with some improvements.

The cab was of Stanier's usual excellent design with the coal bunker built higher than the rear cab windows but angled inwards to avoid them, thus giving good visibility when running bunker first.

Both types of boilers were later modified to carry Adams ‘Vortex’ blastpipe to improve steaming.

They were to be found on various duties – stopping train, suburban passenger routes, branch line, empty stock and banking.