Contemporary large scale interventions, leading ongoing transformation processes of world cities, are often related to mobility systems. Developed around the reconversion or the implementation of infrastructural nodes, they deeply impact surrounding areas within a more general rethinking of urban conditions. However, the relation between urbanity and infrastructure appears at least ambiguous. On the one hand mobility flows represent key components and engines for transformations, on the other hand their physical presence progressively disappears from the urban scene itself, subtly declaring a figurative irreconcilability between the two. Starting from this observation the book, as a contribution for the architectural and urban discourse, reflects on the status of a conflicting relation, with a specific focus on the XXth century, retracing paradigmatic design positions to outline in which ways these hidden infrastructures are actually reshaping the contemporary city scape and which are the opportunities in the architectural research.