In addition to the innovations already adopted for the Modified Hall class, the new class contained several further changes from usual Great Western practice including the use of
double chimneys on certain members and a high boiler pressure of (although this was later lowered in an attempt to reduce maintenance costs). The
boiler was a development that used the tooling for the
LMS Stanier Class 8F boiler, Hawksworth being able to study this design closely when 8Fs were being built at Swindon as part of the war effort. The class initially had a tractive effort of , which was greater than a
Castle Class locomotive, although the tractive effort was reduced to when the boiler pressure was lowered. The class had continuous splashers over the driving wheels and, when named, straight nameplates, making them immediately recognisable from other 4-6-0 classes. They were also fitted with Hawksworth's slab-sided tenders, but the County tenders had a water tank six inches wider than the tenders built for the Modified Halls and retro-fitted to many earlier designs. Some of the early design studies for what became the County included outside
Walschaerts valve gear which would have been a major break from traditional GWR designs. In the event the standard inside
Stephenson link motion of the Churchward and Collett two cylinder classes was used. The
GWR 1500 Class, also designed by Hawksworth, used outside Walschaerts as did the steam railcar units designed under Churchward and the narrow gauge Vale of Rheidol 2-6-2T. ==Production==