For commuter traffic in the south and southeast of London, William Stroudley developed the class A1, of which a total of 50 were built between 1872 and 1880. The tank locomotives with an 0-6-0T wheel arrangement had small driving wheels and were able to rapidly accelerate the commuter trains of that time. Nevertheless, they were able to reach speeds of up to 60 mph (97 km/h) and a sustained speed of just under 50 mph (80 km/h). Because of their small size and barking exhaust noise, they were nicknamed “terriers”.

With the introduction of the more powerful class D1 with 0-4-2T wheel arrangement and the electrification of the first commuter lines, they were no longer needed in the London area. 23 were sold to various other operators and the remainder distributed to rural lines, mainly in Sussex. There they were mostly used in passenger service, sometimes also as push-pull trains. Others were used in shunting duties.

Between 1911 and 1913, 17 A1s were rebuilt into the A1X, becoming heavier and having, among other things, larger cylinders and a longer smokebox. Many survived the Southern Railway and came to the British Railways. Many retirements only came from 1960 onwards. The last ones only disappeared in November 1963 when the Hayling Island Branch was closed.