They were built by Robert Stephenson and Company and were almost identical to the successful Rhymney Railway M class.
The M class and related 1910 introduced R and A class were successful designs ideally suited to hauling heavy coal trains a relatively short distance.
In 1922 the three Neath and Brecon locos were repaired by the GWR and lasted until 1929–1930.
This was because the nature of the work they undertook demanded high adhesive weight, plenty of power with good braking ability, but no need for outright speed, nor large tanks or bunker as the distances from pit to port were short.
All the similar Rhymney Stevenson derived locos passed into British Railways (BR) ownership in 1948.