A derailment, simply put, is the loss of guidance (or contact stability) between any of the rolling wheels and the rail. The wheel may climb up on its flange and over the rail, head to the outside of the track or drop in between the rails. Preventing derailments is a top priority of all transit and passenger rail operators. With a thorough understanding of the causes of derailments and the application of a few simple strategies, the chances of derailments can be significantly reduced. This article will review the causes and potential preventative measures of most derailments.
Before one can begin to map out an effective strategy for preventing derailments, it is essential to understand the mechanism, or the combination of mechanisms, that lead to derailments.
Common Causes of Derailments Operator (Human) Error
Derailments are often attributed, at least in part, to human error. In some cases, the error is associated with a vehicle component or key element of the track structure not being inspected or serviced properly. However, a large portion of those incidents associated with human error can be attributed to operator error.
Most accidents attributed to operator error can be typified to be one of several scenarios. There are those accidents in which one or more trains were operated at speeds in excess of that determined to be appropriate for the conditions at hand. Related to this type of situation, many derailments can be attributed to poor train handling practices, including excessive braking rates. Other derailments, and many collisions, can be traced to the lack of observation or proper interpretation of signals. The manual setting of switches and the coordination of train movements, although not a major aspect of most large-scale passenger and transit operations, can still be a situation in which human error can lead to significant ramifications.
In the vast majority of situations, human error is often associated with one or more parties not following established procedures. However, this is not always the case. Although relatively rare, there are cases in which derailments can result from the lack of clear rules and policies for a given situation. An operator can be particularly susceptible to this situation when major changes to equipment, policies or operating practices are implemented.
Vehicle/Track Component Failure
In many cases, a derailment results simply from the outright failure of a key system component.
From the point of view of the infrastructure, a loss of guidance can be caused by broken or buckled rail resulting from environmental temperature extremes, loss of track fasteners, bolts or broken bolts failing to hold the track gage under load and bridge support or track sub-grade failure. The failure of track components can manifest themselves in track geometry issues such as narrow or wide gage, high track warp, repeated track anomalies such as a series of “bumps” or “dips,” a harmonic variation in crosslevel and deviations in short profile deviations that can result from such issues as failed joints. Operation of vehicles with stiff suspensions over “warped” or “twisted” track can lead to wheel unloading. In addition, switches and other locations of special track work are critical locations at which many issues can arise including switch operator failure or switch point breakage and insufficient compression of spring frogs in special track work.
From the perspective of the vehicle, a broken wheel or axle, bearing failure that interferes with the rolling of the wheels, or any bound suspension components such as the center plates, side bearers or dampers can lead directly to a derailment due to the inhibition of the truck or bogie rotation resulting from a change in track direction. The dimensions of the wheel flanges are also a critical aspect of the vehicle’s condition. Thin flanges, resulting from contact between the flange face and the gage face of the rail, and high flanges, resulting from excessive tread wear, can create problems while negotiating switches.