Tube-gauge steam locomotives may appear to be an unlikely possibility, because of the problems of using such a machine in the confines of a tunnel less than 12-foot (3.7 m) in diameter, but the London Underground had three such vehicles over the years. Two were built by the Hunslet Engine Company in 1899, and the third by Kerr, Stuart and Company in 1922.
The Central London Railway obtained two small steam 0-6-0T locomotives from the Hunslet Engine Company in 1899, to assist with the task of equipping the tunnels once the civil engineering work of building them had been completed. They were numbered 1 and 2, but only the outer wheels had flanges, which enabled them to negotiate curves of 150 feet (46 m) radius. They appeared to have very large side tanks, but of the 1,250 imperial gallons (5,700 L) of water carried, only one fifth was used for feeding the boiler, and the rest for condensing the steam. Boiler pressure was 150 psi (10 bar), which gave them a tractive effort of 12,300 lbf (55 kN).
Although the cabs were wide, headroom was extremely limited, and consequently they were oil-fired, so that they could be operated by one man rather than two. Fuel tanks holding 50 imperial gallons (230 L) of oil were fitted into the bunker, which could also hold 0.75 tons of coal, since the grate was designed so that either fuel could be used. Oil was always used in the tunnels, but coal was often used above ground. Once the railway opened to passengers in July 1900, the locomotives were seldom used in the tunnels, but did occasionally go into them. They were mainly used for shunting in depots and for moving coal wagons at Wood Lane power station, which operated until March 1928. However the two locomotives were scrapped in 1923.
The locomotive became L34 in 1930, and worked on the extension of the Piccadilly line to Cockfosters. During this work it was stabled at either Cockfosters or Arnos Grove. It then moved to Drapers Field, Leyton, to work on the Central line eastern extension. This work was completed in 1949, and the locomotive was then scrapped.