The "Dunalastair" class marked a new era of development in late-Victorian British steam locomotive design.

[2] The average weight of passenger trains had greatly increased in the 1880s and 1890s due to the demand for more comfortable, better-appointed and safer carriages.

The existing pattern of inside-cylinder 4-4-0 express engine was reaching the limits of its development and many railways were resorting to the inefficient practice of double heading to maintain schedules.

In broad design this was identical to the conventional 4-4-0 engine drawn up by his predecessor Dugald Drummond,[2] but it carried a boiler significantly larger than was usual for the time - almost to the full limits that the Caledonian's loading gauge would allow, operating at a relatively high pressure of 160 psi (1,103 kPa).

[2] The class was very successful and developed in four different versions:[5] The rebuilding with superheaters was accompanied by a reduction in boiler pressure and an increase in cylinder diameter.

[6] Classes 766 and 900 were built with eight-wheel bogie tenders with capacities for 4,125 imperial gallons (18,750 L) of water and 4.5 tons of coal.

In the 1930s newer and more powerful LMS locomotives took over their most long-distance duties and the company cascaded the 900 Class to other work.