The addition of a pair of wheels at the rear end of an 0-4-0 afforded the builders to create a larger (and therefore more powerful) locomotive. The extra wheels supported an extended footplate on which could be stored fuel, usually wood.-4-2 wheel arrangement.
We have the only 0-4-2 known on the Mendocino Coast still in our midst. The 0-4-is Daisy who used to belong to the Caspar Lumber Company. She has been restored and now "lives" in the Deli Restaurent across the road from the Skunk Depot. You can read all about Daisy here.