THE TRAIN ORIGINATED IN DENTON (MP 92.5) AND A TERMINAL AIR TEST WAS PASSED BEFORE DEPARTING WITH 5LOADED BALLAST CARS, 6 LOADED FERTILIZER CARS AND 5 EMPTIES. WE WERE TRAVELLING WEST ON A DESCENDING GRADE AT A RATE OF SPEED BETWEEN 5 AND 10 MPH AT (MP 107.8) MY BRAKEMAN LOOKED BACK AT THE CARS COMING AROUND THE CORNER AND SAID "WE HAVE A PROBLEM". I APPLIED THE BRAKES AND SECURED THE LOCOMOTIVE. WE HAD 2 CARS ON THE GROUND AND TORE UP 150 FEET OF TRACK. SUSPECTED CAUSE WAS TRACK PROFILE AND CONTRIBUTING TO THE DERAILMENT WAS A POSSIBLE TIGHT TRUCK.NoneNoneNoneNoneNoneNoneNoneNoneNone