An employee was attempting to install a roller set onto a track system for a rolling-style door on a box railcar. The door was raised and was being supported by a forklift. A sledgehammer was used to hammer the roller set onto the track assembly. The impact of the sledge caused the roller set to slide off the fork, dropping the door 3-4 inches back to its original height. The employee's left index finger was pinched between the fork and a vertical door support pole. The finger was amputated.