Ynyslas is at the southern end of the Cambrian system, between Dovey Junction and Aberystwyth. 9021 had been constructed at Swindon in November 1938 using the frames of withdrawn 'Bulldog' 3411 married to the boiler and cab of condemned 'Duke' 3259 Merlin. It was one of sixteen 'Dukedogs' allocated to Machynlleth and its Aberystwyth sub shed in the mid ' fifties.
The class had been introduced by Collett to work the Cambrian lines, replacing older designs, their light axle loadings making them ideal for crossing the wooden Barmouth Bridge, and they performed well on both freight and passenger duties until January 1960 when 9014 and 9017 the last two survivors on the Cambrian were withdrawn.