The equipment category covers accidents caused by malfunction of some part of the train, including brakes, body parts, the coupler and draft system, truck components, axles and journal bearings, wheels, and locomotive failures.

The track classification includes accidents caused by track, roadbed, or structures. Representative causes include defective track geometry, rail and joint bar problems, damaged switches, switches out of adjustment, and improperly operating signal and control systems.

The human factors category includes accidents resulting from the improper use of brakes, an employee's physical condition, improper signaling or response to signaling, failure to comply with operating procedures, excessive speed, and improper use of switches.

The fourth classification is miscellaneous. This includes accidents caused by events not covered in the other three categories, such as collision with a highway user at a rail-highway crossing; vandalism; shifting load; load falling from car; objects on or fouling the track; snow, ice, or mud on the track; improperly loaded cars; and other factors.