Although its 70 members were only built between 1931 and 1935, the class 6100 was the direct successor to the class 5101 built form 1929 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 since 1905. In general dimensions and driving wheel diameters, these locomotives were still almost identical to the class 5100 of 1903 and the cylinders had the smaller diameter of 18 inches like in the class 5101. By increasing the boiler pressure to 225 psi, it was still possible to increase the starting tractive effort.

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