In late 1920s, train loads were getting heavier as mainline locomotives became more powerful and increasingly capable of pulling larger loads, but the locomotives assembling and shunting these trains in railway yards were struggling to cope with the weight.
The C class as built were fitted with superheated boilers with wide fireboxes and developed 30% greater tractive effort than the Canterbury J class 2-6-0s and with a light axle loading to give the same amount of running rights as the U class 4-6-0s.