The accident occurred when the locomotive was going from bottom shaft to east tunnel (M1796) carbon steel for open head drum had fallen onto the rail track. While the locomotive coming through the carbon steel wire, the flatcar wheels out of the rail track and getting derailed. The locomotive operator quickly applied the brakes and further movement of locomotive was stopped. The tunnel supervisor/ engineer were informed about the incident and necessary steps were taken to put the flatcar back on the track. Normal activities resumed in about 4:00 AM.