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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.

This paper describes the nuXmv symbolic model checker for finite- and infinite-state synchronous transition systems. nuXmv is the evolution of the nuXmv open source model checker. It builds on and extends nuXmv along two main directions. For finite-state systems it complements the basic verification techniques of nuXmv with state-of-the-art verification algorithms. For infinite-state systems, it extends the nuXmv language with new data types, namely Integers and Reals, and it provides advanced SMT-based model checking techniques. Besides extended functionalities, nuXmv has been optimized in terms of performance to be competitive with the state of the art. nuXmv has been used in several industrial projects as verification back-end, and it is the basis for several extensions to cope with requirements analysis, contract based design, model checking of hybrid systems, safety assessment, and software model checking.

Requirement engineering is one of the most important phases in the development process of software and systems. In safety-critical applications, it is important to support the validation of the requirements with formal techniques to identify and remove flaws. However, requirements are often written in textual documents and their formalization and validation is not trivial for non-experts in formal methods. The goal of the OTHELLOPLAY tool is to support formalization of textual requirements and to simplify the use of formal techniques for requirements validation. The tool combines a formal verification engine and the Microsoft Word® editor in a single and consistent environment. A fundamental key in our design approach is a plug-in-based architecture, which uses the Python language in conjunction with a Microsoft Word® Add-In. The user can jump between textual requirements in the Microsoft Word® editor and the corresponding formal requirements model.