Covered hoppers carry dry bulk commodities. If it was indeed a burned-off wheel bearing on a covered hopper that caused the derailment, it was the resulting derailing and burning of adjacent tank cars hauling hazmat liquids/gasses that caused the major disaster. Since there are defect detectors nowadays about every 20 to 30 miles, my question would be why the hot journal wasn't detected (requiring the train to be stopped for inspection) before it ever actually caught fire and burned off. That said, I don't have enough information to do anything other than speculate. Time will tell!