We wish to model railway control systems in a formally precise way so that product lines can be adapted to specific customer requirements. Typically a customer is a railway operator with national conventions leading to different variation points based on a common core principle. A formal model of the core product must be precise and manipulatable so that different feature variations can be specified and verified without disrupting important properties that have already been established in the core product. Cyber-physical systems such as railway interlocking, are characterised by the combination of device behaviours resulting in an overall safe system behaviour. Hence there is a strong need for correct sequential operation with safety "interlocks" making up a process. We utilise diagrammatic modelling tools to make the core product more accessible to systems engineers. The Rail Ground example used to discuss these techniques is an open source model of a railway control system that has been made available by Thales Austria GmbH for research purpose, which demonstrates some fundamental modelling challenges.
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