The potential causes for derailment are by no means limited to the partial or complete failure of the track or vehicle-based components listed in this section. Derailments can result from a combination of conditions in which vehicle or track-based components are within acceptable tolerances but close to thresholds deemed appropriate for identifying suspect components. It is important to recognize the potential for track anomalies that may not meet the threshold used to identify maintenance issues can cause cars to pitch, bounce, yaw or roll in a manner that can lead to vehicle instability, especially in those cases where the anomalies are repeated.
The response of the vehicle to track perturbations is a key aspect to operations and leads directly to the next discussion about important considerations in derailment causes — how the vehicles and track react to each other.
Poor Vehicle/Track Interaction
To make a truly comprehensive effort to prevent derailments, it is essential that the interaction of the track with the moving vehicle, often referred to as vehicle-track interaction (VTI), be considered as a whole.
The successful guidance of a moving passenger rail vehicle, or lack thereof, will be governed by the response of the vehicle to deviations in the track geometry, either by design (curves, switches, etc.) or because of component degradation. In turn, the response of the vehicle to the track is significantly affected by the interaction of each rolling wheel with the rail and the contact conditions between the wheels and rails. A derailment is often the result of many factors combining to create an undesirable VTI situation with not a single cause. For example, a modest track twist (change in crosslevel) near a curve worn switch point could lead to less than desirable wheel/rail contact geometries and the potential for a wheel to climb over the rail, particularly for stiffly suspended trucks.
Several VTI scenarios can lead to derailment, including wheel climb resulting from excessive lateral forces at the wheel/rail interface as compared to vertical forces at the same interface, gage widening and rail rollover, vehicle lateral instability, high wheel loads and their effect on switch components and the forces on the rail that can be generated by hollow worn wheels. The mechanisms of the derailments caused by each of these scenarios can be described as follows: