One locomotive of the Badminton Class was rebuilt with an experimental boiler. With the impending opening of the direct Reading-Taunton route which was of more undulating nature than the route via Bristol, there was a need to ensure that the most appropriate locomotives were provided. Churchward's Chief Assistant, F.G. Wright, designed a large boiler with a very deep firebox, which was fitted to no. 3297 "Earl Cawdor" in July 1903. The boiler was designed to hold a large volume of hot water, forming a reservoir to assist the locomotive running along an undulating line. It also provided a steam space of as against the of the Standard No. 4 boiler, the larger of the two types of standard boiler fitted to the 4-4-0s. This increase in volume was intended to provide a reservoir of steam. The firebox was set deep between the coupled axles, with a horizontal grate. The intention with this was to allow a thick fire to be built up, increasing the area of the firebox in contact with the fire, and decreasing the temperature gradient along the firebox plates, thus reducing the risk of broken firebox stays. The boiler was pressed to . No. 3297 was also fitted with a large cab with two side windows, reminiscent of North Eastern practice. The locomotive was regularly rostered on the most demanding trains, but it soon became apparent that it was not up to the work demanded and it was relegated to secondary duties. In October 1906 the boiler was removed and replaced by a Standard No.4 boiler, the North Eastern style side-window cab having been replaced by a Churchward type in November 1904.