Casey, I would suspect that location would dictate the use of fuel. Where coal is abundant and oil has to be shipped in you will find coal burners. Likewise, if oil is plentiful and coal has to be long hauled then oil will be burned in the firebox. ATSF was a classic example of this with coal burners in the Chicago to Colorado/New Mexico area where coal was easily procured and oil burners in the west with the FT's in the arid in-between - again because water had to be long hauled into the desert for steam engines but not for diesels.