During his time as chief designer at the Barry Railway, John Auld's only design was a tank locomotive intended for heavy coal transport. It was supposed to tow up to 60 coal wagons, each with a payload of twelve tons, from Trehafod to the coast. For this, the locomotive was provided with three coupled axles with the maximum possible axle load and was given a trailing bogie to be able to carry sufficiently large supplies. A Belpaire firebox was used and the smoke box was dimensioned with a view to the future, large enough to later accommodate a superheater. 

In the track systems of mines and ports, a design error quickly became apparent when navigating the narrow manual switches without a locking mechanism. If the locomotive drove into the branching points with the smoke box first, the wheel flanges of the third coupled axle could slip into the point's tongue and move it, causing the bogie to move in the direction pointing straight ahead. Not only could this lead to derailments, but it could also destroy the equalizing pipe between the water tanks, causing water loss and necessitating an emergency dropping of the fire. It also happened that one of the locomotives overturned. As a remedy, the fireman had to get out in such cases and hold the switch lever with all his strength.

Thus, the ten examples of the class in the originally intended role were quickly replaced by the older class B1 and now mainly used for mineral trains. They often hauled trains from Rhymney to New Tredegar on the Brecon and Merthyr Railway and was also found on Great Western lines. Occasionally they were even used as passenger locos on suburban trains to Cardiff. The possibility of installing a superheater in the existing boiler was never taken up, but three locomotives were given a new standard boiler with superheater by the GWR in 1923. Nevertheless, all ten examples were scrapped in 1926.