MxV RAIL R&D, RAILWAY AGE MAY 2025 ISSUE: The Wheel Impact Load Detector (WILD) is an established wayside detector technology used to identify out-of-round conditions, whether due to wheel polygonization or surface anomalies. Conventional WILD systems employ a series of strain gauges installed on the rail web to measure the vertical force exerted by passing wheels. This instrumentation is designed to maximize the portion of the wheel circumference measured over several rotations.
The Association of American Railroads (AAR) Manual of Standards and Recommended Practices (MSRP) Section F, Standard S-61011, specifies the static calibration and validation requirements for conventional strain gauge-based WILDs. As new technologies such as load cells, accelerometers and fiber optics emerge as potential options for determining wheel loads, these alternative WILD systems will require methods for ensuring accuracy beyond a static calibration.
Under the AAR Strategic Research Initiatives (SRI) program, the MxV Rail research team developed enhanced measurement techniques by integrating 1) AAR-qualified, high-accuracy onboard instrumented wheelsets (IWS); 2) a wayside high-accuracy in-track bi-circuit (HAC); and 3) portable rail bumps (PRBs) of varying thicknesses. These elements provide a detailed comparison of the wheel/rail vertical impact force measured both on board and in-track. The MxV Rail research team also conducted preliminary on-track testing to validate the performance of the proposed methods. The objective of the testing was to establish an industry benchmark for validating WILD systems commonly used in wheel removal decisions.
On-track testing was conducted at MxV Rail’s High-Speed Loop (HSL) in Pueblo, Colo. The HAC was installed on the HSL, acting as a surrogate for a traditional strain gauge-based WILD. The test train included a six-axle locomotive, an instrumented passenger car and a loaded 110-ton hopper car with IWS installed at the #1 and #4 axle positions.
For comparison purposes, PRBs were mounted to the rail with the leading edge (the point where the wheel first makes contact) close to the center of the crib (the space between two crossties). Multiple test passes were performed at different speeds and PRB thicknesses. In total, 162 samples with impact forces ranging from a low of 46.5 kips (1 kip = 1,000 pounds) to a high of 97.7 kips were captured by the IWS. The highest recorded force exceeded the AAR “Actionable” limit of 90 kips, thereby warranting wheelset replacement. Additional analysis was conducted to determine the correlation between IWS and
HAC measurements.
Out of the 162 measurements collected, only one instance exhibited a difference greater than 5% when comparing the two systems’ outputs. Even with this outlier, the data analysis demonstrated that 95% of measurements taken by IWS and HAC for the same wheel pass/impact event were within ±5% (Figure 1a).
Figure 1. W/R impact force measurement validation results: (a, top) Difference between in-track HAC and onboard IWS; (b, bottom) Impact forces with speed and bump thickness.
Different PRB thicknesses and test speed variations were successfully used to generate a range of vertical impact loads. Figure 1b illustrates the variation of impact force relative to the speed and bump type, demonstrating that impact loads increased with bump thickness. The trend between impact amplitude and test speed followed a linear trajectory over the speeds tested.
As expected, controlling the PRB thickness and test train speed demonstrated the PRB’s ability to affect impact loads that reach the actionable limit of 90 kips. An IWS can serve as an onboard benchmark in WILD validation, and the combination of “IWS + PRB” presents a viable verification solution.
MxV Rail is currently working with industry partners to develop recommendations for updating AAR Standard S-6101. This work includes formalizing procedures for the newly developed approach and outlining relevant limitations to ensure comprehensive validation of WILD technologies.
“Leveraging emerging technologies supports the North American rail industries’ long standing pursuit of continuous improvement in the area of equipment health monitoring,” said Hark Braren, Director of Mechanical Reliability at BNSF Railway. “Further, the integration of these new technologies will further enhance the industry’s ability to ensure safety for the railroads, their employees and the communities they serve.”
