Caledonian Single No. 123

For the 1886 World's Fair in Edinburgh, the Caledonian Railway had a single locomotive built with seven-foot wheels that would be able to travel at a very high average speed. The 4-4-0 of the class 66 by Dugald Drummond was used as the basis, in which the rear coupled axle was replaced by a trailing axle and the remaining driving axle was equipped with larger wheels. In order to be able to compensate for the low adhesion mass, a modern sanding system was used. Although a steam-powered system was just being introduced at the time, a different approach was being pursued here. Because Caledonian used Westinghouse air brakes instead of the vacuum brakes common in Britain, the compressed air was used to power the sand.

After the World's Fair, locomotive number 123 was used in express service on the West Coast Main Line in Scotland. In the years 1888 and 1889 it got caught in the middle of the “Race to the North”, in which various railway companies on the east and west coasts unofficially tried to find the fastest journey time from London to Edinburgh. With often only two or three passenger cars, the locomotive did very well despite the inclines. On the 100 miles long section from Carlisle, average speeds of over 50 mph were regularly reached, on one occasion even 59. In addition to being used in front of heavier trains in combination with another locomotive, it was also used for special tasks. These included passenger trains for the Caledonian's inspectors and directors, and also repeatedly used as the lead locomotive for the royal family's train when they were en route to Balmoral Castle. The locomotive was taken over by the LMS in 1923 and was not withdrawn until 1935, making it the last British express locomotive with only one driving axle. From 1958 it ran again in front of special trains and is now in the Transport Museum in Glasgow.