This paper describes experience with developing and maintaining a requirements tracing model for a system of systems for Positive Train Control (PTC). Positive Train Control is a system of monitoring and controlling train movements to provide increased safety. A PTC system consists of four segments: locomotive, office, wayside, and communication. We have found that it is useful to describe the segments of a PTC system as a system of systems. A requirements tracing model has been developed to manage the requirements of a PTC system of systems. The tracing model provides traces from requirements, design, features, and test artifacts across the development lifecycle. Tracing models are required for safety-critical systems, and they are very useful for performing various types of impact analyses on a PTC development project. The characteristics of the tracing model are described and lessons learned with implementing the model are discussed.

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