Caledonian Railway Single No.123 is a preserved Scottish steam locomotive.

The unique 4-2-2 was built by Neilson and Company in 1886, works No. 3553, as an exhibition locomotive. It entered London, Midland and Scottish Railway service in 1923 and the LMS renumbered it 14010 and gave it 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The locomotive was withdrawn in 1935, by which time it was the last single-wheeled express engine running in Britain and set aside for preservation.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.