Vehicle/Track Interaction
Union Pacific has deployed a Vehicle/Track Interaction (VTI) system to identify and monitor high vertical and lateral hits on the system. The VTI system utilizes off-the-shelf equipment from ENSCO, Inc., to measure car body vertical and lateral loads above the suspension system, along with two axle-mounted accelerometers to measure the vertical loads at the suspension system. The system is currently mounted on 10 locomotives (two types) and one coal hopper — a car type that experienced 36 top chord failures within a six-month period. By distributing the units across the system, a VTI-equipped car or locomotive sees the coal corridor every seven days and primary intermodal corridors every 10 days.

"The VTI system represents a paradigm shift in that we are looking at things that don’t necessarily measure out as FRA defects," said Dwight Clark, UP's Director Track Maintenance - Technology Engineering.

The VTI system is designed to:
• Detect vehicle and track interaction deviations.
• Provide a proactive approach to reducing damage to vehicles and track.
• Improve the track inspection process.
• Quantify and prioritize the exceptions.
• Prevent costly service and equipment failures.
• Evaluate the effectiveness of maintenance activities.

UP has been able to find problems with joints, crushed heads, which generate high hits, and stressed or damaged stringers at bridges. The system has also been able to detect multiple perturbations that are not discernable from visual inspection, but can excite the car body enough to warp the top chord of coal hoppers.

"Not every VTI hit is a track standard exception," Clark said. "But we know that by addressing them we can prevent these areas from growing into exceptions."

VTI data is downloaded into UP’s Track Maintenance Planning system where the hits and their magnitude can be monitored for growth. Satisfaction with the system is such that UP plans to purchase 10 additional units (nine locomotive-mounted, one car-mounted) yet, this year.

Norfolk Southern began to look into the performance characteristics of a particular car type — cross-trough coil steel cars — after a couple derailments. Using WILD and TPD data, along with vehicle dynamics simulation and analysis and physical testing at the TTCI, NS determined that unbalanced loads could generate lateral loadings of up to 15 kips per axle. NS found that wheel unloading occurred at 17 kips at about 24 mph.

"Running under balance speed with the load shifted to the low rail side represents the worst-case condition," said Walter Rosenberger, Operations Engineer in the NS Research and Tests Department.

Wheel bearing failure also has been on the short list of vehicle-related issues to be addressed. Extensive work has also been done to identify the acoustic signatures of bearing faults. While the initial work that was done in the mid-90s focused on package bearing units, with an outer shell, rolling elements and cones, the rolling stock in current use utilizes a wide variety of bearing types, each with the potential for large variation in the rules for detection, said Uwe Kopke, Principal Engineer, Vipac Engineers & Scientists, Ltd.

"This work is complementary or similar to hotbox detectors. It provides a predictive maintenance tool that allows railways to predict early faults and plan for maintenance," Kope said.