Water truck parked at end of shift. During am hours, short occured at battery cable. spark ignited grease on frame grease dripped onto fuel line, melting small hole in line. Diesel dripped onto ground. Sparks ignited diesel on ground. Passing coal train saw smoke underneath truck @ 5:30 am called wright fire dept. fire dept arrived @ 6 am. extinguished fire @ 6:03 am.