The GWR 455 Class, also called the "Metropolitan" or "Metro" Tanks, was a series of 140 2-4-0T 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.

Sixty "Metro" Tanks were built, from 1868 onwards, during the lifetime of their designer, Joseph Armstrong.

His successor William Dean regarded the class so highly that he would add a further 80, the final 20 examples appearing as late as 1899.

A proportion of the class was intended to be fitted with condensing apparatus, for service on the Metropolitan: Nos.

However, photographs show that some engines in the series did not in fact carry the apparatus.

The GWR also provided about half the trains on the Hammersmith & City, and much of the Metropolitan's Aldgate-Paddington-Richmond service.

Typical centres were Swindon, Oxford and Gloucester, while others were also seen in South Wales and in the West of England.

They even hauled main-line trains on cross-country routes such as Gloucester-Cardiff, being capable of remarkably fast running when needed.