For over a decade, the benefits of Formal Methods software system development techniques have been realized in safety critical applications in a variety of industries, such as aerospace, aviation, and of course rail-transit. Many of these Formal Methods techniques have focused on the development of new systems. In new system development, Formal Methods provide assurance of error-free specification, design, and software code implementation, often via automated code generators. Legacy systems have been largely excluded from the Formal Methods field, due to a perception that their use would entail costly re-engineering, require specialized knowledge, and would necessitate a large learning curve.

However, Formal Methods may be used for Verification and Validation (V&V) of legacy systems without the costly risk factors enumerated above. A description of the methodologies used is provided, including a description of how the process is tailored to new acquisitions and deployments of legacy systems. The modeling was fully integrated in the rail property and contractor development processes. As each design phase progressed, the formal methods process provided an increased level of confidence with regard to safety assurance and the correctness of the system design. This was achieved by generating formal proofs for each safety critical function, and by refining these formal proofs into progressively lower-level elements. These proofs then provided criteria for design reviews, code reviews, and test scenarios. The benefits of obtaining formal, that is, mathematical, formula-based, proofs of system design and consistency is indisputable. The criteria for evaluation are objective and thorough. One need not rely on the adequacy of peer reviews and manually generated test cases. Formal Methods V&V is therefore a cost-effective process that provides proof of correctness early in the system development life-cycle.

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