A hybrid testing methodology for railway control systems

International standards for V&V processes prescribe systematic testing as a fundamental step of safety-critical systems life-cycle, in order to prove the fulfilment of their requirements. However, proposed approaches are quite general and, for complex systems, imply an excessive number of test-cases to ensure the correctness of system behaviour in any operating scenarios, including unexpected ones. A more detailed methodology is needed to extensively test all the aspects of a complex system, while keeping the number of test-cases below a reasonable threshold. This paper describes the ASF hybrid testing methodology, combining black-box and white-box techniques, based on the identification and reduction of influence variables. Such an approach was successfully applied to validate ASF implementation of the SCMT system (an Italian Automatic Train Control specification), showing its time effectiveness and full achieved coverage. The same methodology, with the related customization, is now being improved in order to test the new ERTMS/ETCS systems.