I think most of these cars are copies of those made by the "Differential Dump Car Company" of Findlay Ohio.
I visited Findlay Ohio in 1992 (I think, it was during George H.W.Bush's unsuccessful re-election campaign, and CSX were running a Presidential Train through Ohio. After checking a few locations, we picked Findlay as a good spot to shoot the train.)
The factory was still there and there were a number of dump cars sitting around in the yard. The street to the entrance was "Differential Avenue".
The NSW tramways purchased two(?) self propelled Differential Dump cars which were black banned during the great depression since they reduced the need for labourers to unload ballast.
I think there are a number of variations on these cars with the Pilbara Iron Ore companies, including some original "Difco"s and some Goninan cars.