Recent gage restraint measurement system (GRMS) developments include the redesign of GRMS vehicles using a deployable split-axle instead of a freight truck mounted split-axle. This new test configuration results in boundary condition changes in the applied loads and split axle location, which influence test results. To ensure the equivalence of test results from these two systems, a comprehensive evaluation of the mechanistic basis for previous GRMS rulemaking was conducted and coupled with a fundamental investigation of factors influencing GRMS performance and inspection accuracy followed by field-testing to verify conclusions. Comparison tests between the original GRMS vehicle design and the redesigned vehicles identified the need to enhance the current gage widening ratio (GWR) equation to accommodate the increased range of vertical test loads represented by the different GRMS vehicles. GWR has been the leading source of test result discrepancies between the original GRMS design and redesigned vehicles over the same territory. The discrepancy between the inspections likely resulted from the increased range of vertical test loads represented by the distinct test vehicles, since GWR has treated vertical load as a constant. The Gage Widening Projection (GWP) parameter was proposed to replace GWR as a result of an ongoing investigation. GWR was originally developed as an indicator of fastener and tie performance by providing an extrapolated total gage widening deflection at a limiting load condition. Testing at the limiting load condition is not conducted because of the potential for damage to track components. A deflection at a lower load is extrapolated based on conservative track load-deflection behavior to the limit load, which represents an extreme but not unexpected gage widening event. The concept behind both GWP and GWR is similar. However, the GWP parameter includes vertical load as a variable, where GWR treated vertical load as a constant. The large variation in vertical load represented by the various test scenarios requires the consideration of variable vertical load in the extrapolation process to ensure an equivalent basis for inspection and safety. Based on analytical modeling and field-testing, the GWP parameter was found to perform more consistently between vehicles than GWR. A limiting condition based on a combination of deflection and load was selected to provide comparable inspection results and safety for both parameters. Additional testing has been conducted to further evaluate the data and indicates excellent performance of the GWP parameter, and perhaps merits further consideration regarding the limits.