The River class was Smith's only design for the Highland Railway. They were the most powerful locomotives of this company, but exceeded the axle loads permitted there. Although Smith had designed the locomotives to induce as little hammer blow as possible on the rails, they were not allowed to be used and Smith was forced to resign.

However, they managed to sell all six locos to the Caledonian, where the rails were stronger. After minor changes to the loading gauge, they were successfully used in front of express goods trains and were subjected to a great deal of stress. After the takeover by the LMS, the calculation of the hammer blow was better understood and it was concluded that these locomotives could certainly run on the lines of the former Highland Railway. This in fact took place before the locomotives were retired between 1936 and 1946.