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The main objective of the signalling system ERTMS/ETCS is to permit safe and interoperable operation of European railway traffic [6]. Safe traffic requires precise data about the position and the movement of trains. This paper focuses on the train positioning system of ERTMS/ETCS level 2. This system is based on the combination of the odometer and the electronic position markers (balises) that are detected by the trains during its travel. This paper describes how this positioning system works and derives the different causes of its failures. An elementary Coloured Petri Net model of the positioning system is presented. This model is validated through simulations. For safety reasons, the model shows that the train has to stop if it misses a balise or if there is a high risk of odometer errors. This confirms the recommendations of the Subset 026-3.