To improve the running characteristics of underground locomotives running between Moorgate Street and the South of London, Patrick Stirling introduced the 0-4-4T wheel arrangement in 1872. In contrast to other cases, where an additional axle was used to mount a bigger boiler or to increase the supplies, the new class 120 was even lighter than the previous class 126 0-4-2T. 48 had been built by 1881.
No. 533 after the rebuild to a crane
flickr/stratfordman72
As they had to work in underground tunnels, they had a condensing apparatus. In the 1890s, many got a new domed boiler, as can be seen on the photo. When others were rebuilt around 1907, they lost their condensers and were converted into well tank locomotives. In 1905, number 533 got a special rebuild when it was converted into a crane locomotive for working at Doncaster Works. 