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.
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:
- Wheel Climb
While a train wheel or wheelset rolls along the track on its tread, some guidance is provided by the steering ability of the coned or profiled tread as the track changes direction. However, it is the flange of the wheel ultimately contacting the gage face of the rail that really forces the wheel to follow the rail and change direction. The forces generated between the wheel and the rail can be resolved into two components — the lateral force (L) and the vertical force (V). These force components are illustrated in Figure 1.
In order for a wheel’s flange to climb up the gage face of a rail and over the rail head to the outside of the track, the wheel lateral and vertical forces must be such that the vertical force that acts to keep the wheel on the rail is overcome by the lateral force and the friction forces that exist between the wheel’s flange and the gage face of the rail. The ratio of lateral force to vertical force that must be exceeded in order for the wheel climb scenario to exist is specified by Nadal’s Limit, a criterion that is defined as follows:
where ì is the coefficient of friction between the wheel and the rail and a is wheel flange angle as illustrated in Figure 1.
- Gage Widening and Rail Rollover
The problems that can be encountered with excessive wheel/rail interaction forces are not limited to wheel climb scenarios. For instance, if the lateral force generated by the flange contact between the wheel and the rail is relatively high, this force can cause lateral rail displacement. This rail displacement produces what is known as gage widening and can lead to a wheel/rail separation as shown in Figure 2.
Rail rollover, as shown in the left side of Figure 3, is one of the most common sources of accidents especially when the vehicle travels over the spiral transition between tangent, or straight, track and the full body of a curve. The critical value of L/V for rail rollover is approximated by the ratio D/H with D and H defined as per the illustration on the right side Figure 3. In general, if the moment generated by the lateral force is higher than the moment generated by the vertical force, the rail can rotate about its corner.
- High Wheel Loads and Their Effect on Switch Components
As a wheel passes over the switch point and the tip of the stock rail, a high lateral pressure on the tip of the switch point will be generated. This can lead to battering of the switch point and the eventual fracture of the stock rail. This situation, in combination with thin, high flanges, can lead to the scenario where the wheels can get caught on the switch point and allow the wheel to ride up on the switch components.
- Vehicle Lateral Instability
The wheels of rail vehicles are tapered in order to allow for the wheelset to steer down the track. Small lateral displacements of the wheelset will result in the steering of the wheelset back and forth between the rails in an oscillatory or “swaying” motion. When that oscillatory motion of the wheelset and the vehicle continues to grow in amplitude as the vehicle moves down the track, the vehicle is said to be experiencing lateral instability.
For each vehicle there is a specific speed known as the critical speed, above which the vehicle will exhibit lateral instability. This speed is highly dependent on the characteristics of the vehicle suspension systems, mass and mass moment of inertia of the vehicle bodies, and the wheel profile. In general, track irregularities push the wheelset laterally. If the vehicle speed is above its critical speed, the wheelset lateral motion has the tendency to grow. The lateral motion of the wheelset will be limited only by the wheel flange coming in contact with the gage face of the rail. In the case of severe lateral instability, high forces can result from the impact of the wheels on the rail that can lead to damage of the track and create conditions that could eventually lead to derailments.
- The Effect of Hollow Worn Wheels
When a wheel exhibits significant wear on its tread to the point where a rut is formed around the circumference of the wheel around the center of the tread, it is said to be a hollow worn wheel. A typical measurement of hollow wheel tread is taken as the vertical depth of the lowest point on the tread of the wheel from a straight edge established as the plane of the original unworn design tread taper. This measurement procedure is illustrated in Figure 5.
Two conditions of concern can exist with hollow worn wheels. As the tread wears, the tip of the wheel flange “moves” further away from the top of the rail, creating a condition where the top of the flange is considered to be high and can strike switch components. Hollow worn wheels also adversely affect the steering of the truck and can increase the potential for derailment. A hollow tread can result in a shift of the wheel/rail contact location to the field side of the rail, thereby increasing the amount of vertical force that is located outside the gage of the rail. This situation can significantly increase the potential for rail roll and ultimately to a rail rollover derailment.
Strategies to Consider for the Prevention of Derailments
There is no easy way to provide complete protection from derailment. It requires commitment and diligence on the part of the operator to implement and adhere to sound practices aimed at minimizing or eliminating those factors that can contribute to a derailment scenario. Based on the discussion presented to this point, the following points are offered for consideration during the development of a derailment prevention strategy:
1. No industry can fully eliminate the occurrence of human error. In an effort to minimize these errors, operators need to be vigilant in enforcing their training requirements for all personnel. Just as critical, however, is the review and refinement of training and procedures. Operators must be able to objectively analyze and, when necessary, modify policies and training when issues arise to mitigate the chances of human error leading to a derailment.
2. A critical component to the prevention of derailments is reliable track and vehicle inspection practices. In the past, operators have responded to reports of excessive car body motions over locations that are exhibiting indications of failure by issuing slow orders and reporting them to the track department for attention before a derailment occurs. Not only is this approach reactionary in nature, but it is of little use in areas with low track speeds, in which locations of concern will result in a lack of significant car body motion. In cases like this, the operator is typically unaware of the hazard until the train derails.
Operators should consider being proactive in developing their inspection strategy. Detection of incipient component failures should be a top priority for operators. The measurement of the response of the vehicle to track can be a very effective means of identifying track locations of concern.
3. Railroads and transit operators must also aggressively perform maintenance as required. An operator with optimal inspection practices are only part way through an effective derailment prevention strategy. To that end, the following suggestions for key aspects of a maintenance strategy are offered:
A) Proper wheel flanges must be established and maintained. Depending on the design and maintenance of the truing machine, newly trued wheels can have “shallow” flange angles on the order of 60 degrees. Such wheels will most often wear to steeper flange angles (72 to 75 degrees) within 100 to 200 miles of operation. If they do not encounter any adverse track conditions in the meantime, they will be fine. However, if one of these wheels with a shallow flange angle encounters a low joint or a brand new switch point (with a “scaly” or “rusty” high-friction surface), a derailment can result. American Public Transportation Association (APTA) strongly recommended in its “Passenger Rail Equipment Safety Standards Task Force Technical Bulletin 1998-1” that organizations develop or adopt maintenance and inspection practices that ensure that a minimum sustained flange angle of 72 degrees to provide a margin of safety to protect against derailment. Assessment of the wheels following truing is a key aspect of this recommendation.
B) In addition to the level of attention that is required for wheel flanges, the operator should routinely inspect wheels for flaws that can lead to flats. The inspection process should also identify wheels exhibiting hollow worn treads for remediation to reduce the risk of gage widening and rail rollover. Restoration of the wheel profile can also lead to improved vehicle stability and hence reduce the potential for derailments due to lateral instability.
C) Those responsible for vehicle maintenance should maintain suspension clearances and springs to assure trucks (bogies) equalize properly. It is important to note that some truck designs are more sensitive to spring problems than others. Manufacturer’s tolerances should be followed closely. It should also be pointed out that some newer truck designs, such as those from Europe and Asia, are typically less tolerant to track warp or twist. They often include chevron, rubber springs, which are stiffer than “equalized” truck designs common in older designs.
D) Lubrication of center plate of the track will reduce the generated resisting moment to the steering moment during curve negotiation. The use of proper lubrication will reduce the required force for steering and hence reduce the tendency of derailment in curves.
4. The subject of friction at the wheel/rail interface was briefly discussed earlier in this article. Indeed, friction management is a topic worthy of several articles on its own. A sound program established to address rail wear and friction throughout an operation — either through track-based systems, vehicle-based mechanisms or a combination of both — can be a critical element of a successful derailment prevention strategy.
5. In addition to being aggressive about performing maintenance on track infrastructure upon awareness of issues, additional steps can be taken with the right-of-way that can provide additional protection against derailments:
A) Limit warp to no more than one inch in 10 feet (or less depending on axle spacing associated with the vehicles on the system) and no more than two inches in 60 feet (truck center spacing) to protect against wheel unloading.
B) Install guard rail on sharp curves to avoid wheel climb. Guard rail can also reduce the gage wear of high rail. In general, the guard rails are installed inside of the low rail. For very sharp curves, it is recommended to install guard rails on both the inside and outside rails.
6. The characteristics of a vehicle’s suspension system are critical to its susceptibility to wheel unloading and derailment. A soft primary suspension will provide better steering of the truck over curved track, hence reducing applied lateral forces on the track that cause derailment. However, the use of very soft primary suspension may affect the vehicle’s lateral stability. Optimized characteristics of the primary suspension may be required in case of high-speed trains.
7. Perhaps the most important component of a derailment prevention strategy is the proper attention to VTI management. This is a subject that should concern both track and vehicle maintainers, but overall responsibility is often an issue. In most cases, it’s split between track and vehicle maintenance and operations. Regardless of who is responsible, it is extremely important that someone in the operations “own” the responsibility for VTI-related issues.
Brian Whitten is a senior staff engineer and manager of high speed rail programs with Ensco. Eric Sherrock is a staff engineer and serves as the program manager for Ensco’s operations, maintenance instrumentation and analysis support contract for the Federal Railroad Administration’s Office of Research and Development. Dr. Khaled Zaazaa, a staff engineer with Ensco, is the co-author of the book Railroad Vehicle Dynamics – A Computational Approach. After 33 years as an active member of the rail safety and R&D communities with Ensco, Kevin Kesler left in September to launch NexRail LLC.
More Related Information:
Archived Article: Rail Modernization- It’s Not Your Father’s Rail Vehicle
Archived Article: Global Rail Safety: Equipment and Technologies
Mass Transit Buyer’s Guide: Railroad Right-of-Way, Maintenance-of-Way Equipment