[4] The performance of the 1894-built Jones Goods Class used on passenger trains during summer showed the viability of larger locomotives.

For financial reasons, the new locomotives, which were intended primarily for heavy express trains, were delivered in small batches at intervals of several years.

Nevertheless, the Castles were able to meet the essential requirements and could haul trains of up to 200 tonnes without a pilot locomotive over the Pass of Drumochter and the Slochd Summit on the shortcut route.

At the outbreak of the First World War, the Castles were also responsible for hauling passenger trains to Thurso and Wick, carrying naval officers and seamen for the Grand Fleet based at Scapa Flow.

By the time the Second World War broke out in 1939, ten locomotives had been withdrawn, and in 1945, examples from all batches were still in service.

The last example in service, 14690 Dalcross Castle, was withdrawn from Aviemore Sheds in 1947, and scrapped at Inverness a few weeks later.

[5] They were also fitted with slide valves, as the Highland Railway had negative experiences with the piston valve-fitted Loch Class locomotives they had purchased in 1896.

Other minor changes included thicker bottom rings[clarify] for the firebox and reinforced drive rod bearings.

Christopher Cumming, Drummond's successor, used slightly larger driving wheels, which also increased the wheelbase.

[2] From 1916 onwards, the Highland Railway gradually fitted the earlier batches with new boilers, as the existing ones had become worn out due to poor maintenance during the war.

[13] Due to their performance being less satisfactory compared to other ETAT 4-6-0s, they were soon used primarily on secondary services, such as in front of construction trains.

[13] In 1907, NBL built six more Castle Class lookalikes (with more differences than their Scottish and French counterparts, such as the use of the Belpaire firebox) for the Caminhos de Ferro Portugueses da Beira Alta.