The ashpans form hoppers down in between the frame rails and between the wheels. On engines with trailing trucks, the hoppers are on each side of the trailer truck axle. There are sliding doors on the bottoms of the hoppers that allow you dump the ashes out the bottom.
You do have to take a bit of extra care to clean out the trailer truck journal boxes on occasion to make sure you are not filling the box with grit and crud. We used to put some extra cotton waste in the box just inside the lid to catch the extra dirt, replacing it once a week.