Architecture and safety requirements of the ACC railway interlocking system

The paper summarizes the design, implementation and validation of the Computer Based Interlocking (ACC) designed by Ansaldo Trasporti under the supervision of the Italian Railways. Particularly, details are provided on the methodology that has been adopted to set the safety requirements and to give preliminary safety demonstrations. During the development of the ACC design, new drafts of European Standards have been circulated on the design and validation of railway signalling systems. Safety assessment activities compliant with the new European Standards have been started and integrated within the ACC safety process. The failure models presented in this paper have demonstrated that the ACC design requirements assure the system compliance with the required safety integrity levels. They also have allowed for the identification of the parameters to be measured by fault injection tests in an exhaustive experimental evaluation environment, which will be developed in the subsequent phases of the validation process.