For the Metropolitan and District Lines in London, Joseph Armstrong designed the class 355, also known as “Metropolitan Tanks” or simply “Metro Tanks”. It was created with a driving wheel diameter of just five feet to ensure adequate acceleration. Since the underground tunnels were also to be run through, many locomotives were fitted with a condensation device. No cab was installed, as Armstrong believed it would have reduced staff attention.

The first 60 machines were built between 1868 and 1878 in three batches of 20 each. In the first two batches, all axles were mounted in the inside frame, in the third batch the leading axle was in external bearings and the wheelbase of the coupled axles was slightly larger. In the early years, the locomotives were also required to haul goods trains within London city limits until the class 633 became available. After Armstrong's death, a total of seven more batches with a total of 80 more locomotives were procured by William Dean between 1878 and 1899. These had a larger boiler and larger supplies.

In the 1880s, the first batches were already being rebuilt with a larger boiler and larger water tanks. Half and fully enclosed cabs were retrofitted in later years. The first of the older locomotives were already retired around 1900. However, the newer ones in particular were used for a longer period of time.

With the electrification of the Metropolitan and District Lines in 1905 and 1907, the locomotives were also used on other lines. For this purpose, the condensers were removed and partially a push-pull train control was installed. For the lines of the Underground, the GWR kept about 50 available until about 1930, which were then replaced by the class 6100 Large Prairies. Other locomotives were used in more rural regions and some were even used on the main line. In the thirties most were retired. A total of ten were still in existence when British Railways was founded in 1948, but these disappeared by 1949.