A new "add-on" technology has been developed to relate measured track geometry to vehicle performance on a real-time basis. The technology can also be used on historic track geometry data to examine the effect of track geometry degradation on vehicle performance.

Following several years of research and development, the Transportation Technology Center, Inc., Pueblo, Colo., has started the demonstration and implementation of its performance-based track geometry, PBTG, inspection system. (1) The PBTG inspection system is a new add-on technology that can be implemented on conventional track geometry inspection vehicles.

The PBTG inspection method is an improvement over the current track geometry inspection method because track geometry defects, identified using current methods, do not always relate to vehicle performance (Figure 1). Implementation of the PBTG inspection method should lead to prioritized track geometry maintenance based on vehicle performance. As such, railroads can expect to reduce potential derailment incidents and the stress state of track structure caused by poor vehicle/track interaction.

The PBTG technology allows users to specify vehicle types sensitive to track geometry input (such as empty tank cars, loaded hoppers and gondolas), as well as a variety of operating speeds for vehicle performance analysis. On a real-time basis, this technology identifies track segments that may produce poor vehicle performance and generates recommended track geometry maintenance actions.
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Vehicle performance and PBTG exceptions

Performance-based track geometry inspections relate measured track geometry to vehicle performance. Vehicle performance can be defined in terms of different vehicle response parameters relating to derailment potential, stress state of track structures and vehicle ride quality. These vehicle responses can be defined as follows:

* "Derailment potential" is often defined by L/V ratio (lateral/vertical wheel or axle load ratio) and "vertical wheel unloading" values with respect to allowable limits. The L/V ratio criterion is intended to prevent flange climb derailment and the vertical unloading criterion is intended to prevent wheel lift derailment.

* "Stress state" is often quantified by the "maximum vertical and lateral wheel loads" exerted on track.

* "Ride quality" is often quantified by "vertical and lateral accelerations on the car body."

The current version of the PBTG inspection system defines vehicle performance in terms of derailment potential and stress state, though it can be expanded to include ride quality analysis. More specifically, the current PBTG system can be used for vehicle performance analysis for the following parameters: single wheel L/V ratio, vertical wheel load and lateral wheel load.

"PBTG exception" is defined as a track geometry condition within a track segment that will likely cause an undesirable vehicle response. Unlike a track geometry defect, defined by the FRA Track Safety Standards (2) or railroad maintenance standards, which is point specific, a PBTG exception is segment specific. It includes combined and multiple geometry deviations within a segment and takes into account the effect of track features, such as curvature and spiral.

As an example, Figure 2 shows where a combined track geometry condition led to a poor vehicle response (L/V ratio above 1.0). As shown, the combined track geometry deviations (cross level, alignment and surface) were located between 4,500 feet and 4,700 feet. The presence of an entry spiral also contributed to the poor vehicle response.