A mixed-traffic locomotive is one designed to be capable of hauling both passenger trains and freight trains. The term is mostly used in the United Kingdom and those nations following British practice.
Mixed-traffic locomotive Wikipedia
Mixed-traffic locomotives were common in the early years of railway development, but changing traffic patterns meant that later designs would tend to be more specialised, to haul the fastest passenger trains and the heaviest freight trains.
Driving wheel sizes were generally intermediate: larger than the small wheels of most freight locomotives, required to produce a high tractive effort at low speeds, but smaller than the large wheels express passenger locomotives required to reach high speeds.
The term was generally only used for steam locomotives, since most of the first generations of British mainline diesel locomotives were equally capable of hauling both passenger and freight trains, specialised designs not appearing for many years.