Product line models of large cyber-physical systems: the case of ERTMS/ETCS

A product line perspective may help to understand the possible variants in interactions between the subsystems of a large, cyber-physical system. This observation is exemplified in this paper by proposing a feature model of the family of ERTMS/ETCS train control systems and their foreseen extensions. This model not only shows the different components that have to be installed when deploying the system at the different levels established by the ERTMS/ETCS standards, but it also helps to identify and discuss specific issues, such as the borders between onboard and wayside equipment, different manufacturers of the subsystems, interoperability among systems developed at different levels, backward compatibility of trains equipped with higher level equipment running on lines equipped with lower level equipment, and evolution towards future trends of railway signalling. The feature model forms the basis for formal modelling of the behaviour of the critical components of the system and for evaluating the overall cost, effectiveness and sustainability, for example by adding cost and performance attributes to the feature model.