Although the 70 examples were only built between 1931 and 1935, the class 6100 was the direct successor to the class 5101 built up to 1949. It was developed specifically for use in the London area as a replacement for the class 2221 4-4-2T tank locomotives built from 1905 onwards. In general dimensions and coupling wheel diameters, these locomotives were still the same as the 1903 class 5100 and the cylinders had the smaller 5101 cylinder diameter of 18 inches. With the boiler pressure increased to 225 psi, the starting tractive effort was increased by around 12.5 percent despite the reduced heating surface.

For use on the suburban platforms in London-Paddington, some locomotives were fitted with mechanical train stop, which opened a valve when passing a stop signal and thus triggered emergency braking. The typical area of application ranged from north-west London to Aylesbury, which corresponded to a distance of around 40 miles. Even after the founding of British Railways, the engines remained indispensable, but they soon faced strong competition in the form of the first generation of diesel railcars. Although their status fell as a result, the last examples were able to hold their ground in service until 1965.