Eleven examples of this final express passenger class designed by Patrick Stirling for the FSWR were built at Kilmarnock Locomotive Works at regular intervals between March 1865 and July 1868 (Works Nos. 32, 39-40, 43-4, 47-8, 50-53). They were numbered 45, 151-6, 84, 61, 16 & 79. The members of the class were fitted with domeless boilers and safety valves over the firebox. The safety valves were later replaced by those of Ramsbottom design over the centre of the boiler following a boiler explosion at Springhill in 1876. They were fitted with Stirling’s own design of cab and open fan-like splashers.
