'Pug' locomotives are small steam locomotives which were produced for light shunting work, usually at dockyards, factory sites, steelworks, collieries, and other similar locations. The name is derived from a common term in Scotland for a small industrial shunting engine, typically an 0-4-0 tank. Whilst most commonly used for small shunting engines, on some railways the term 'Pug' was used for all tank engines. For example, the very large Glasgow and South Western Railway 540 Class 4-6-4T express passenger locomotives were known to their enginemen as the 'Big Pugs'.