In 1903, the Caledonian Railway had no passenger locomotives larger than 4-4-0s, and the heaviest trains over its main line between Glasgow and Carlisle required to be double headed, even in the less demanding southbound ('up') direction.

[4] By 1906, experience with the 49 Class had enabled McIntosh to design an improved version, and the installation of new turntables at major engine sheds presaged the arrival of five new locomotives.

[5] Neither class was equipped with superheating when built, but all seven locomotives were rebuilt with Schmidt superheaters and new cylinders during 1911.

These modifications reduced coal consumption but made little difference to the locomotives' performance,[2] and McIntosh built no more large passenger 4-6-0s (although he did build smaller 4-6-0s for goods traffic).

The two 49 class locomotives were extensively renewed around 1924 with new frames and cylinders,[6] but all of the Caledonian passenger 4-6-0s were quickly eclipsed by new LMS Compound 4-4-0 and Royal Scot 4-6-0s.