EMPLOYEE WAS A PASSENGER IN AN OPEN TYPE TRACK VEHICLE (PERSONNEL CARRIER). A PIECE OF LOOSE COAL RIB APPROX 1 1/2' X 1 1/2' X 6""THICK FELL FROM THE TROLLEY WIRE SIDE RIB AS THE VEHICLE PASSED BY STRIKING THE PASSENGER ON THE SIDE OF HIS RT KNEE, CAUSING HIS KNEE TO BEND INWARD.