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Belgium is characterised by a centrifugal federalism in which the federated entities constantly seek to maximise their autonomy. In this context, the governance of certain public issues, such as mobility, is highly complex. Based on the case of the project for an RER in Brussels, the paper shows firstly that questions of mobility lead to the formation of a space of rivalries. This project is at the heart of numerous conflicts of usage, of visions of the city and also around the practical implementation of mobility policies. Secondly, we go on to explore the way this project is politically regulated, by questioning the institutional and sectoral borders, and the rights to use the rail resource.