Analytical Model-based Evaluations of Railway Applications: a Case Study on Sensitivity Analysis

1 Personal use of the material in this paper is permitted. However, permission to reprint or republish this material for advertising or promotional purposes or for creating new works for resale or redistribution, or to reuse any copyrighted component of this work in other works must be obtained from the authors of this paper. Summary This paper reports an experience made in building a model and analysing the dependability of an actual railway station interlocking control system developed by Ansaldo Trasporti: the ACC CIS (Computer Interlocking System). Despite our decision to restrict the analysis to the Safety Nucleus subsystem a considerable effort was necessary in order to cope with complexity and size. We have identified a modelling strategy based on a modular, hierarchical decomposition which allow the use of different methods and tools for modelling the various level of the hierarchy. This paper focuses on the analyses made in order to deal with some of the simplifying assumptions made in the models. The approximation error resulting from a given assumption (in a selected set) has been estimated by comparing the results obtained by releasing the assumption itself. This comparison allowed us to select those simplifying assumptions which did not invalidate the results of the models and to focus on those which required a more careful and detailed modelling effort. The paper reports some examples of the extensive analyses performed towards the validation of the assumptions made.