Caledonian Railway 72 Class

The Caledonian Railway 72 Class was a series of inside-cylinder 4-4-0 express passenger locomotives designed by J.F. McIntosh and introduced in 1906, representing the final development of the Dunalastair design lineage before the introduction of larger 4-6-0 express types on the Caledonian Railway. The 72 Class incorporated a superheater — relatively early adoption of this technology on a British express locomotive — alongside the enlarged boiler proportions that McIntosh had developed through the Dunalastair I, II, III, and IV sequence, giving a locomotive that combined the free-steaming large-boiler heritage of the Dunalastairs with the thermal efficiency advantages of superheating.

McIntosh had observed with interest the experiments with superheating being conducted on the Prussian State Railways by Wilhelm Schmidt in the early 1900s, and the 72 Class represented his practical application of superheating to the Caledonian's express passenger fleet. The superheater, which passed the steam through tubes inside the boiler flues before delivery to the cylinders, significantly increased the steam temperature and reduced its moisture content, allowing the same boiler pressure to produce more work from the same quantity of coal and water. The practical effect was lower fuel and water consumption per mile — a significant operating economy on the long Anglo-Scottish runs.

The 72 Class worked Caledonian express services from 1906 alongside the Cardean 4-6-0s and the Dunalastair IV 4-4-0s. They gave reliable service through the Grouping period and into the early LMS era. None was preserved, but the 72 Class is historically significant as one of the earlier British express locomotive classes to demonstrate the operational benefits of superheating in main-line passenger service.