The GWR 455 Class, also called the "Metropolitan" or "Metro" Tanks, was a series of 140 2-4-0 tank locomotives built for the Great Western Railway, originally for their London suburban services, including running on the underground section of the Metropolitan Railway, the source of their nickname. Later on the class was seen on many other parts of the GWR system. Sixty "Metro" Tanks were built, from 1868 onwards, during the lifetime of their designer, Joseph Armstrong.