Compound steam engines use steam more than once, first at high pressure, then extracting more energy from the steam by expansion in larger diameter lower pressure cylinders. This improved the efficiency of steam engines and in railway locomotives permitted modestly sized locomotive to deliver greater power. In Great Britain the only compound railway locomotives to have long working lives were the Midland Railways' 4P class. These were nicely balanced engines which could be usefully employed on the former Midland mainline express services and on many secondary and cross-country services throughout the LMS period.
Elsewhere in Britain technology improvements in locomotive cylinder and valve design through the early 1900s allowed 'simple expansion' engines which used steam in just one cylinder cycle to approach the efficiency of the compound engine in railway service.