In 1904, the LSWR Locomotive Superintendent, Dugald Drummond, was tasked by his superiors to analyse the possibility of an updated version of his successful T9 class 4-4-0.

With open competition against the Great Western Railway in earning revenue from ocean-going traffic in the south-west of England at Plymouth, there was a need for a new design of powerful locomotive capable of hauling heavy loads at high speeds.

The last in an unbroken lineage of Drummond 4-4-0s stretching back to his unsuccessful C8 Class of 1898, the L12 continued the tradition of solid construction and robust operation.

[1][page needed] The major design difference between this and the Class S11 was the fact that the 6-foot-7-inch (2.007 m) driving wheels seen on the T9 were reinstated for fast running on the LSWR main line.

[2][page needed] The boiler was also similar to that of the T9s, capped off with a dome and stovepipe chimney, though the smokebox was of a smaller design in comparison due to the initial lack of superheating.

[1][page needed] One major modification was made by Robert Urie, who exchanged the saturated steam boilers for the superheated variety, resulting in an enlarged smokebox when compared to that fitted on the S11 Class.

[1][page needed][5] 421 was one of two locomotives hauling a boat train to London Waterloo from Plymouth, which failed to round a curve at the eastern end of Salisbury station and subsequently derailed.