In the very early days, locomotives were generally considered either "goods" or "passenger", but, of course, there were some cargoes - which can loosely be described as "fast freight" - that could command a locomotive of either type: fish and milk, for example, was carried at higher speed and lower density than coal or stone. By the middle of the 19th century, there were many locomotives that would be used for either freight or passenger work - the LNWR "coal tank", built for local coal trains, ended up in use on local passenger trains more than anything as its brakes proved inadequate for coal traffic. Whilst a locomotive designed for heavy freight would be unlikely to be found on a passenger service and a locomotive designed for top express passenger trains would be unlikely to be found working freight, there was a large intermediate area of locomotives which could be and regularly were used either for light freight or secondary passenger work.