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Relay Interlocking Systems (RIS) are analog electromechanical networks traditionally applied in the safety-critical domain of railway signaling. RIS consist of networks of interconnected components such as power supplies, contacts, resistances, and electrically-controlled contacts (i.e. the relays). Due to cost and flexibility needs, RIS are progressively being replaced by equivalent computer-based systems. Unfortunately, RIS are often legacy systems, hard to understand at an abstract level, hence the valuable information they encoded in them is not available.In this paper, we propose a methodology and a tool chain to analyze and understand legacy RIS. A RIS is reduced to a Switched Multi-Domain Kirchhoff Network (SMDKN), which is in turn compiled into hybrid automata. SMT-based model checking supports various forms of formal analyses for SMDKN. The approach is based on the modeling of the RIS analog signals (i.e. currents and voltages) over continuous time, and their mapping in terms of railways control actions. Starting from the diagram representation, we overcome a key limitation of previous approaches based on purely Boolean models, i.e. the presence of spurious behaviors. The evaluation of the tool chain on a set of industrial-size railway RIS demonstrates practical scalability.

We present the EuRailCheck tool, which supports the formalization and the validation of requirements, based on the use of formal methods. The tool allows the user to analyze the requirements in natural language and to categorize and structure them. It allows to formalize the requirements into a subset of UML enriched with static and temporal constraints for which we defined a formal semantics. Finally, the tool allows to apply model checking techniques specialized for the validation of formal requirements. The tool has been developed and validated within a project funded by the European Railway Agency for the validation of the European Train Control System specification. By now, the tool has been successfully used by about thirty railway experts of different companies.