Based on the Prairie locomotives later known as the class 3100 and class 5100, the GWR had 41 of a heavier variant built as class 3150 in 1906. The standard type 2 boiler was replaced with a larger type 4 boiler, which was mounted slightly higher here. Due to the higher weight, the axle load was assigned to the “Red” category instead of “Blue”, which reduced the road availability of the locomotives compared to their predecessors. However, the main suburban lines were strong enough to allow the use of these locomotives. So they hauled the heavy commuter trains on these lines. They were withdrawn between 1957 and 1960.

As early as 1938, the then chief engineer Charles Benjamin Collett had started a rebuilding program of five locomotives, which were to be used as helpers on ramps. These had a driving wheel diameter reduced by five inches to 5 ft 3 in (1,600 mm), an increased boiler pressure of 225 instead of 200 psi and a cylinder diameter that had been increased by half an inch. This increased the tractive effort by more than 20 percent. These were known as Collett class 3100, but no more than these five were built.