Innovative Track Monitoring System Using Optical Fiber Sensors: Measurement of Applied Wheel Load in the Field Condition

Abstract: In railroad tracks, non-uniformity in the track substructure leads to variations in the wheel–rail contact force, where poorer support conditions result in larger applied wheel loads. To evaluate the condition of track support, researchers and railroad agencies have been monitoring the magnitude of the applied load and reaction force in the track structure. Various monitoring systems, including load cells and half- or full-bridge strain gauges, have been employed for this purpose; however, these systems can only capture local conditions for individual ties or spans.
A recent research study at Oklahoma State University introduced the use of optical fiber sensors (OFS) in field applications, enabling multiplexing within a single cable to obtain measurements at multiple locations along the rail. This approach integrates multi-point measurements of axial strain with beam theory to correlate the rail response with the applied load. The key innovation is the capability to measure applied loads on multiple adjacent spans simultaneously under a moving loading, allowing continuous mapping of track support conditions and identification of problematic ties or spans.

To validate this monitoring concept, a field test was conducted on a four-span track segment at the Transportation Technology Center (TTC) in Pueblo, Colorado. In this validation test, a quasi-distributed form of the OFS was used to measure axial strains on the rail under variable support conditions created by removing a tie. The monitoring test plan was to interpret the applied wheel load from the axial strain measurement. The results demonstrated that the OFS-based monitoring system effectively captured the variability in tie conditions by measuring the applied loads across individual spans.