The Caledonian Railway 721 Class (the ‘Dunalastair’ class) was a class of 4-4-0 steam locomotives designed by John F. McIntosh for the Caledonian Railway (CR) and introduced in 1896. All survived to be absorbed by the London, Midland and Scottish Railway (LMS) in 1923 and some survived into British Railways (BR) ownership in 1948. The weight of trains increased in the ‘80’s and ‘90’s due to better-appointed and safer carriages. With rising passenger volume and competition between railways engineers needed to produce engines that could operate longer, heavier trains at higher speeds. McIntosh provided the solution with the 1896 ‘Dunalastair’. Although identical to the 4-4-0 engine of his predecessor Dugald Drummond, it carried a larger boiler with more fire tubes of a greater diameter than before. While top speed was the same as other 4-4-0’s the more productive boiler gave an improved ability to maintain high speed with heavy loads on steep gradients, allowing the ‘Dunalastairs’ to set high average speeds over the arduous main line across Beattock Summit. The principles of the 721 Class and its boiler were adopted by many other locomotive engineers and railways in the 1890 s and early 20th century, leading to the so-called ‘big engine’ period of design. Other engine classes such as the GNR Class C1 and the GWR 4100 Class were inspired by the success of the “Dunalastairs“.