The London Underground began operation in 1863 as the Metropolitan Railway and expanded through the 19th century, serving stations as far as 80 km from the centre of London on a combination of underground and surface tracks. Propulsion was entirely by steam engines, and staff and passengers complained about the foul atmosphere and soot emitted by these locomotives. It was not until 1890 that the first electric locomotives were introduced on the new City and South London Railway, and all of the system was not electrified until the first decade of the 20th century.
The Metropolitan Railway A Class and B Class were the primary locomotives used by the underground during the steam era. They were equipped with condensers so they did not emit used steam into the tunnels, which would have raised their humidity to intolerable levels. They did, however, still exhaust flue gases into the tunnel, burning anthracite coal or coke to reduce the amount of soot expelled.
After electrification of all of the Underground lines by 1906, the steam engines were withdrawn from service and sold or scrapped. Some of them remained in use in surface railways until 1936.