This paper describes a methodology for the analysis of the passage of red signals by train drivers (SPAD). The methodology considers all the components that support the train driver in performing his activity and analyses the role of the interactive systems, such as computer based equipments, to understand possible human-machine interaction problems. The application of the methodology has shown that a strong involvement of the users of the interactive systems is a key element to identify their real operative usage, and that this is sometimes quite different from the one hypothesized by the system designers. The paper reports the results of the analysis of an incident to illustrate the application of the methodology.