Based on the Prairie locomotives later known as the class 3100 and class 5100, the GWR had 41 units of a reinforced variant made as the 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 area of use of the locomotives compared to their predecessors. However, the main suburban routes were string enough to allow the use of these locomotives. So they pulled the heavy local trains on these routes. The machines were retired between 1957 and 1960.
As early as 1938, the then chief engineer Charles Benjamin Collett had started a rebuilding program of five engines, which were to be used for pushing on ramps. These reduced the coupling wheel diameter by five inches to 1,600 mm, increased the boiler pressure from 200 to 225 psi and increased the cylinder diameter by half an inch. This increased the tractive effort by more than 20 percent. These machines were known as the Collett class 3100, but the number remained as five.