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.
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.
[1] They were mainly used for shunting in depots[2] and for moving coal wagons at Wood Lane power station, which operated until March 1928.
At the same time, the line was extended southwards to Morden, and the railway bought an 0-4-2ST from Kerr, Stuart and Company in 1922, to assist with the equipping of the extension.
The driving wheels were 30 inches (0.76 m) in diameter, and it was considerably less powerful than the Central London machines, generating a tractive effort of 6,500 lbf (29 kN).