Compound locomotives, where steam is used more than once to improve overal efficiency, were briefly popular around the end of the 19th century. The system worked by sending steam at high pressure from the boiler into one set of cylinders, then exhausting the steam at a lower pressure into a larger cylinder where more energy was extracted from the expansion of the steam.
The Midland Railways' 3-cylinder class 4P compounds were the only successful type built in Britain. Compound engines were difficult to get right, as ideally the work donw by the high and low pressure cylinders should be as equal as possible, Achiving this in a marine engine which ran at steady speeds was easy, but locomotive engines must be worked efficiently across a range of speed. As the development of better valve gears and events for simple locomotives improved their efficiency the cost and complexity of compound engines negated the savings in coal consumption.