In the context of the development of the trans-European transport networks (TEN-T), the stated objective of successive policies has been to improve the interconnectivity between national networks and to link Europe’s “margins” to the continent’s political, economic and demographic center. However, these margins have been redefined by the successive enlargements of the Union, as well as by the diversity of understandings of what constitutes marginality, turned into a political argument, on all scales. Proceeding from the results of interviews conducted with private and public stakeholders, and from the observation of conferences organized by the main lobbies, we will attempt to demonstrate how the concept of “margin” has been exploited in a narrative legitimizing projects or demands. We will present a comprehensive overview of the sets of arguments based on the idea of margin, defined herein in its geometric, social and economic, and political acceptation (it becomes a justification, an element of legitimation of the demands or infrastructure projects), and we will question their discrepancy with a scientific approach of the concept.