They were an extended version of the Whale's Precursor Class 4-4-0, with slightly smaller driving wheels.

A 19in Express Goods Class with smaller driving wheels was also built from 1906.

In 1915, 1361 Prospero was experimentally rebuilt with four cylinders, Dendy Marshall valve gear and superheated.

All entered London, Midland and Scottish Railway (LMS) stock upon grouping in 1923.

In 1934 the thirteen then remaining were renumbered with the addition of 20000 to their numbers make room for Black Five, Patriot and Jubilee class locomotives.