What is "compound locomotive"

A compound locomotive is a steam locomotive which is powered by a compound engine, a type of steam engine where steam is expanded in two or more phases. Colloquially referred to as a compound engine, the locomotive is only one application of the compounding principle.

Compounding became popular for railway locomotives from the early 1880s and by the 1890s were becoming common. Large numbers were constructed, mostly two- and four-cylinder compounds, in France, Germany, Austria, Hungary, and the United States. It declined in popularity due to maintenance issues and because superheating provided similar efficiencies at lower cost. Nonetheless, compound Mallets were built by the Norfolk and Western Railway right up to 1952.