The design started out as an 0-4-0 type but was found to have a pronounced tendency to waddle. The addition of the rear wheels helped with that to some degree and I think also allowed the installation of a larger fuel bunker. The "D" class was the same idea, more or less, just with the truck on the front instead of at the back.
They must have been out of government service fairly early since the letter "C" was re-used for the 2-6-2 switchers later on.