The following is an interim progress report covering the development work on a prototype Vertical Track Stiffness Measurement system (VTSM). The Federal Railroad Administration’s Office of Research and Development has begun development of a prototype, dynamic track stiffness measurement system to quantify the relative strength of railroad track in the vertical plane. This effort is design to promote improved railroad safety through quantitative analysis of the strength of track structures. This report provides a review of the technical foundation for the proposed system, reports the results of Phase 1 laboratory testing, and outlines the remaining steps necessary to complete Phase 1 development and to obtain a working prototype. To date, VTSM development efforts have progressed from an initial feasibility study through the selection of a design concept, and initial testing and development of a functioning prototype. To develop and refine data collection instrumentation and software, laboratory experimentation tasks have been completed. These tests have determined the ultimate system resolution and sensitivity to conditions that may be present in the field. These results, detailed in this report, indicate that the design concept should provide adequate resolution and accuracy for making dynamic displacement measurements, and the system is ready for field testing. In addition, the report describes the proposed field testing procedure for the prototype VTSM. The field test will be made using purchased components as well as a significant amount of contractor-supplied equipment. Also, the MMID Railway will contribute motive power, rolling stock, and crew personnel and track time to support these field test activities. Field tests will determine if the prototype system can make accurate and repeatable dynamic vertical stiffness measurements.