In the Whyte notation for the wheel arrangement of locomotives (primarily steam locomotives, an 0-6-0 is a locomotive with six powered driving wheels (thus three powered axles), and neither leading or trailing wheels.
The 0-6-0 was generally considered a switcher (UK: shunter) type, since its lack of guiding axles meant that it wasn't too stable at speed, yet its lack of unpowered axles meant that all weight was adhesive weight and thus tractive effort was maximised. The 0-6-0 became possibly the most common wheel arrangement for switchers worldwide; the 0-4-0 proved to be too small and not powerful enough for most applications, while the 0-6-0 proved to be 'just right'. 0-8-0 and larger switchers were popular, but many smaller and lighter railways could not use them.
In the United States, huge numbers of 0-6-0 switchers were produced. An 0-6-0 was the smallest of the USRA Standard classes designed and produced during the brief Government control of the railroads through the USRA during World War I. 255 of them were built and ended up in the hands of about two dozen US railroads; in addition, many of them (and others) built numerous copies after the war. The Pennsylvania Railroad rostered over 1,200 0-6-0 types over the years, which were classed as type B on that system. US 0-6-0s were generally tender locomotives.
In England, 0-6-0 tank engines were the common shunting locomotives on all railways throughout the twentieth century. All of the Big Four companies to emerge from Grouping in the 1920s used them in large numbers, and the Great Western Railway had an especial liking for the type, especially in pannier tank form, a type they produced until Nationalisation in 1948. When diesel shunters began to be introduced, the 0-6-0 type became the most common also; many of the British Railways shunter types were 0-6-0s, including Class 03, the standard 'light' shunter, and Class 08 and Class 09, the standard heavier shunters.
In addition, all the Big Four companies employed 0-6-0 tender locomotives for slower freight work.
Continental European railways also used 0-6-0s, though possibly not in the proportions used in England; they were generally reserved for light switching requirements, station pilots and the like, and on very small branch lines. Larger tank engines were used for most branch line service.