Applying System-Theoretic Process Analysis (STPA)-based methodology supported by Systems Engineering models to a UK rail project

Systems safety in railways focuses on providing the necessary assurance that the railway system is operationally safe and meets all relevant regulatory requirements. Safety risks associated with changes in the UK railway are controlled through the Common Safety Method for Risk Evaluation and Assessment (CSM-RA). As part of the CSM-RA framework, various safety analysis methods such as Failure Modes and Effects Criticality Analysis (FMECA), Fault Tree Analysis (FTA), Event Tree Analysis (ETA) and other traditional analysis methods conducted via expert brainstorming such as Hazard Identification (HAZID) workshops have been relied upon for many years in the UK rail industry; aiming to evaluate and mitigate all reasonably foreseeable hazards. This paper reports a comparison case study of the application of a novel System-Theoretic Process Analysis (STPA)-based methodology against the traditional approach for hazard analysis in UK rail projects. The proposed methodology uses Systems Engineering (SE) models in each of its steps. The application of the novel methodology demonstrates that it is suitable for hazard identification and analysis in complex rail systems. It shows that the approach goes beyond the capabilities of traditional methods, provides insights into the interaction among system components and captures hazards within the context of the whole. The SE models used in this study prove to be valuable not only for illustrating the System of Interest (SOI) visually, but also providing a high-level understanding of the system and a more detailed understanding of component interactions. They also improved the focus, in scope, effectiveness, and efficiency of the analysis.