The Caledonian Railway Locomotive Works were originally at Greenock but moved to St. Rollox, Glasgow, in 1856.
The locomotive classes are listed under the names of the railway's Chief Mechanical Engineers.
Hence earlier numbered classes could well have appeared later in time.
Until the appointment of Dugald Drummond, unlike most other British railways, almost all engines had outside cylinders, and the 0-6-0 arrangement was quite rare, goods engines being of type 2-4-0 or 0-4-2.
The LMS built some locomotives to Caledonian Railway designs after 1923.