Caledonian RailwaySingle No. 123 is a preserved Scottishsteam locomotive. The unique 4-2-2 was built by Neilson and Company in 1886, works No. 3553, as an exhibition locomotive. In 1914 it was placed on the Caledonian Railway duplicate list, and renumbered 1123. It entered London, Midland and Scottish Railway service in 1923 and the LMS renumbered her 14010 and gave her the power classification 1P. During the 1920s it was allocated to working the directors' saloon, but it was returned to ordinary service in 1930. The locomotive was withdrawn in 1935, by which time it was the last single-wheeled express engine running in Britain, and set aside for preservation.
Restored to steam by British Railways in 1958, it ran railtours and enthusiast specials until the end of steam in Scotland.
Tri-Ang released a model of No.123 in the 1960s, in Caledonian livery and labelling. This model shared its chassis with the model of the Dean Single released at the same time, and was powered by its single driving wheel. Following the other styles of modelling at the time, the model was produced up until the 1970s. Later it was revived in the 1980s in LMS Crimson livery, and was subsequently released in a limited-edition pack in its Caledonian colors. In 2007, the locomotive was released in LMS Black livery, with DCC capability being added to allow the locomotive to fit the then current range. A train pack entitled 'The Last Single Wheeler' was introduced in 2010 featuring the locomotive in LMS Crimson livery again, with three LMS carriages of the time.