INJURED IS A MOTORMAN. INJURED TRANSPORTED A 630 SCOOP LOADED ON A CARRIER TO 7 WEST TAIL TRACK. AFTER STOPPING THE LOCOMOTIVE & LOADED CARRIER, THE CARRIER ROLLED BACK AGAINST THE LOCOMOTIVE CAUSING IT TO START SLIDING DOWNHILL. THE LOCOMOTIVE & CARRIER SLID APPROX 6 XC BEFORE DERAILING. INJURED WAS THROWN AGAINST OPERATOR CAB.