Those "porches" on the later wagons are spillage deflectors. They protect the airbrake reservoirs, triple valves and handbrake linkages from coal ingress when the loader operator at the mine bins gets too enthusiastic or misses his cue to close the bin doors. with this design there is no room to fit this gear under the wagon between the bogies. The longitudinal doors offer a better discharge flow with less coal left in the wagon after dumping than for transverse doors.
The porches also reduce the the amount of wind resistance and air turbulence between the wagons when the wagons are running at line speed.
Unanderra in oz