In the early days the ubiquitous 0-6-0s of both tender and tank design were 'general utility' engines performing all duties except express passenger services. By the end of the nineteenth century most railways were designing locomotives for more specific work and at the beginning of the twentieth century the need for faster freight overnight services led to the design of engines with larger wheel diameter that were not only fit for these services but for slower passenger services also, whilst the need for smaller wheeled engines for the mineral and other heavy freight services created the need for more specialist locomotives for that activity. Most railways moved from the 4-4-0 design for this 'general utility' work to the 2-6-0 or 4-6-0 wheel arrangement and the introduction of Churchward's 43XX mogul in 1911 was recognised as a truly 'mixed traffic' locomotive. Large numbers of similar locomotives were then produced by most of the pre-Grouping railways and developed by all the Big-Four from 1923. The author here describes the basic design, history, operation and performance of all the mixed traffic classes of the former Great Western and London Midland and Scottish Railways and their predecessors, engines of the LNWR, Caledonian, Highland and Glasgow & South Western Railways in particular. Mixed traffic locomotives of the LNER, SR and BR will be described in a second volume.