Managing Wheel/Rail Interaction on Rail Transit Systems
By Bob Tuzik • January, 2008
All railways must deal with operating/engineering and vehicle/track interaction issues. But rail transit systems, which operate under the microscope of the urban environment, face special, often unique, challenges. Speakers at Rail Transit ’07, Advanced Rail Management and Interface Journal’s third seminar devoted to wheel/rail interaction on rail transit systems, identified some of the issues that transit properties face and some of the ways they’ve found to deal with them.
The Southeastern Pennsylvania Transportation Authority (SEPTA) and its consultant Zeta-Tech Associates, Inc., for example, examined two low-speed, wheel-climb derailments on its Market Frankfort Line. While factors such as rail type and operating parameters differed, wheel surface roughness from newly trued wheels played a role in both derailments. A change in wheel profiles from SEPTA’s M3 profile (which has a 1:38 conicity and small radius in the flange/throat area) to the M4 profile (which has a 1:20 conicity and large radius in the flange/throat area) also was a factor in the derailments.
Onsite investigations and NUCARS simulation indicated that in the first derailment, wear (caused by the new M4 wheel profile) on the gauge corner of the stock rail led to a mismatch with SEPTA’s older M3 wheel profile. This significantly increased the lateral forces and led the wheels with the M3 profiles to “walk off.”
The surface roughness of newly trued wheels with the M4 profile was a contributing factor in the second derailment. Worn switch points provided a ramp for the new wheel, which under high lateral forces and high surface roughness caused the wheels to climb.
Investigations also showed that wheels and rails on SEPTA wear into a compatible pattern over time. A drastic change in the wheel/rail interface, however, disrupts the compatibility and can result in derailments. Even at that, looking at wheel and rail profiles alone, is not enough, said Donald Holfeld, Zeta-Tech’s general director of field engineering. “You must also consider factors such as wheel gauge and track gauge and how they work together to become compatible.”
The Washington Metropolitan Area Transit Authority (WMATA) installed a wayside Wheel/Rail Load Detector (WRLD) in order to identify poorly performing vehicles and prevent wheel climb derailments.The WRLD, which was supplied by the Transportation Technology Center, Inc. (TTCI), indicates truck performance by measuring lateral and vertical loads, and angle of attack (and corresponding values such as speed, average car weight and total train weight). Working with its consultants, Booz Allen Hamilton and the TTCI, WMATA found the WRLD to be an effective early warning device that can identify potential “troublemakers.”
“The wayside monitoring system allows us to identify poor performers and select individual vehicles for inspection and unscheduled preventive maintenance,” said WMATA Vehicle Engineer Mike Hiller. This is useful in evaluating the dynamic/curving performance of individual vehicles and the fleet, overall. Through use of the WRLD system, WMATA expects to increase truck life by 10% – 25%, and wheel life by 50%. More significantly, it expects the WRLD system to eliminate wheel-climb derailments.
Another type of Wayside Wheel Condition Monitoring (WCM) system, developed by Teknis Electronics, was implemented at RailCorp’s CityRail 1,300-mile mixed freight/passenger network in Sydney, New South Wales, Australia. While this type of technology is well established in the freight industry, the implementation of a WCM system on a suburban network identified unique problems and requirements associated with mixed traffic.
The Teknis WCM system uses a combination of dynamic (accelerometers) and static (strain gauges) measurements to detect wheel defects at line speeds. In this application, rolling stock engineers worked closely with repair facilities to manage maintenance requirements, determine the root cause of the defects and develop early intervention strategies. The process has yielded a reduction in the number of wheel defects by a factor of 30, said Keith Bladon, Teknis’ managing director.
This system can isolate and trend small defects, identify out-of-round wheels and analyze damage potential and the causes of re-emerging wheel defects. It can also be used to monitor, plan and schedule maintenance requirements. “We’ve seen that 95% of wheel defects start small,” Bladon said.
Some wheel defects on passenger railroads and rail transit systems are a result of seasonal adhesion issues, caused by leaves on the rail in the fall. Slip-slide events caused by poor wheel/rail adhesion during these periods generate wheel flats, which increase maintenance requirements and reduce equipment availability. Research has shown that wheel/rail friction levels affect the extent of the damage to each wheel during a low-adhesion wheel slide, said Graham Curtis of DeltaRail Products, a supplier of technology addressing low-adhesion conditions. Left untreated, wheel flats generate out-of-round wheels, which are difficult to detect.
While the problems relating to seasonal adhesion issues in the UK are similar to those in the U.S., approaches to dealing with them differ. Railways in the UK typically employ systems such as Wheel Slip/Slide Protection (WSP) systems and equipment such as the SmartSander system to control wheel slip and braking efforts. By equipping a fleet of 100 Electric Multiple Units with the SmartSander system, for example, one UK railway reduced station overruns by 70%, and delay minutes by 73%, Curtis said. The low-adhesion incident rate dropped from 0.3 incidents per train per year to 0.08 — a 73% decrease. Wheel truing requirements were reduced by 60%. The SmartSander equipment has been adapted for use in the U.S. and is currently undergoing service testing on Metro-North.
Vehicle design and maintenance practices also impact wheel/rail interaction and derailment potential, as Peter Klauser, a vehicle dynamics engineering consultant, pointed out in a presentation on “the Effects of Secondary Suspension Air Spring Imbalance on Wheel Climb Derailment Potential.” Air spring imbalance typically results from a combination of static imbalance due to car weight distribution, leveling system defects, and the effect of track twist. The result, Klauser said, is that truck side vertical loads are increased or reduced in rough proportion to the diagonal variation in air spring pressures. And since air spring stiffness is roughly proportional to air spring pressure, the car can be supported on “stiff” springs at two corners and on “soft” springs at the opposing corners. Under dynamic conditions, this may result in further unloading. Wheel unloading, combined with high lateral forces due to curving, leads to increased wheel climb derailment potential.
Klauser discussed results of a study of wheel unloading due to air spring load imbalance and its effect on derailment potential. Vehicle dynamic response and an actual derailment were modeled using a vehicle dynamics simulation (VAMPIRE®). The vehicle model was based on a detailed model of a car equipped with powered two-axle articulated frame trucks. Typical worn wheel and rail profiles were used.
A nominal case was established to evaluate the response of the vehicle without air spring load imbalance, but with a realistic amount of carbody weight imbalance. Consulting engineers modeled the individual and combined effects of air spring load imbalance, truck rotational resistance and wheel/rail friction on a single wheel. The results showed that a combination of factors can significantly increase single wheel L/V ratios and the risk of derailment.
Air spring load imbalance alone is seldom enough to lead to a wheel climb derailment, Klauser said. But when combined with other causal factors, such as the coefficient of wheel/rail friction, flange geometry and operational factors (such as the effects of stopping in a spiral before proceeding at low speed through the body of a curve), air spring load imbalance will significantly increase the potential for wheel climb.
Measurement is an essential aspect of optimizing wheel/rail interaction on Wiener Linien, the Vienna (Austria) urban transit system. Markus Ossberger, assistant program manager of Wiener Linien’s Department of Construction and Subway Planning, described how Wiener Linien closely monitored wheel/rail contact as part of an integrated rail grinding strategy. The strategy focused on measurement data to monitor wear and the grinding process, the capabilities of the grinding equipment and the experience of the staff that controlled the grinding program.
Even before the program began, however, Wiener Linien examined its track and vehicle designs and vehicle steering capabilities. The railway also determined the limits of accuracy regarding wheel and rail profile measurement, and overall optimization.
“Precise measurements and comprehensive databases provide tools that allow engineers to manage wheel and rail assets — particularly in systems with grooved rails,” Paul Mittermayr, Managing Director, Mittermayr Scientific Consulting GmbH, Bureau of Applied Mechanics and Mathematics, said regarding methods that can be used to optimize rail (and wheel) profiles in the urban rail context.
Wiener Linien varied rail profiles to improve contact geometry. By distributing the contact patch locations, the railway was able to keep the equivalent conicity within a favorable range, which improved rolling radius difference and vehicle curving capabilities. As a result of regular measurement and rail-profile analysis, Wiener Linien is able to better evaluate rail conditions and plan grinding programs. It is also better able to estimate the remaining rail life on segments of the system.
Overall, Ossberger said, “preventive grinding with precisely defined quality standards may reduce the grinding budget by 10 to 20 percent.” Preventive grinding has also reduced the number of rail fractures from more than 30 to six per year.
While all agree that optimization is a good thing, wheel/rail interaction is hard to see. To do so, Gordon Bachinsky, president of Advanced Rail Management, presented an animated three-dimensional picture of what wheel/rail interaction looks like during various operating conditions. During wheel/rail contact, rails undergo successive rolling, lateral, and vertical motions, while the wheelsets undergo successive lateral and yawing motions, Bachinsky said. “You can’t really see or address lateral or vertical motion, or flange contact’s effect on wheel/rail contact at the tread surface.” An animated three-dimensional look makes it easier to understand the importance of having a properly matched rolling radius differential in order to promote healthy steering in curves.
Measurement is also required to ensure that good contact geometry is maintained between third rail power systems and vehicle-mounted collection systems. Ta-Lun Yang, vice president of Ensco Inc., discussed a third-rail measurement system that has been deployed by the Beijing Metro, China. In order to ensure smooth contact of the vehicle-mounted contact shoes and the third-rail power source, geometry is referenced to the position of the running rails.
Beijing Metro recently deployed a modern track inspection vehicle, installed on a custom-built metro coach to perform automated track inspection. The inspection system, which includes full track geometry and third-rail geometry, uses image-based sensors for rail positioning and inertial-based reference for track geometry. The inspection car is used on existing lines as well as on a new line equipped with linear induction motor technology.
Research and experience drawn from North American freight railways is being adapted and applied to passenger and transit railways. James Dwyer, senior transit consultant with STV, Inc., presented an overview of the research tasks carried out under the Transit Cooperative Research Program (TCRP) Project D-7. The TCRP research, which is done in conjunction with the Association of American Railroads and the TTCI, has generated reports that address specific needs of rail transit agencies. Research topics include broken rail detection, wheel/rail friction, flange climb derailment criteria, and wheel/rail profile design and maintenance guidelines, among others.
William Moorhead, principle of TRAMMCO, LLC., reported on a joint effort between the American Public Transit Association (APTA) and the American Railway Engineering and Maintenance-of-Way Association (AREMA) to develop recommended practices for embedded track for rail transit.
“We’re doing this because manuals and design guides relating to ‘best practice’ designs and construction methods are not up to date,” Moorhead said. “As a result, the rail transit industry puts a lot of time, effort and money into projects with disappointing outcomes and future maintenance problems.”
Excessive noise and vibration have long been sources of complaints in the urban rail environment. While a number of methods, such as friction management, and control of wheel/rail profiles are used to mitigate noise and vibration, the use of so-called mass-spring-systems effectively reduces the transmission of vibrations directly at the source. The mass-spring system uses concrete track slabs supported on a resilient interface. Soft steel coil springs in the mass-spring-systems attenuate vibrations and reduce decibel levels. Since the concrete slabs in the system are relatively low depth, they can be used in tunnels, at grade and on elevated tracks, said Hans-Georg Wagner, GERB Schwingungsisolierungen GmbH & Co.’s head of Building and Trackbed Isolation. The system has been shown to effectively reduce decibel levels by 20 dBA or more.
There is no shortage of challenges. But rail transit systems are obtaining a better understanding of what can be done to better manage wheel/rail interaction in the urban environment.