This article focuses on some salient issues of urban sustainable development in France, specifically regarding to urban means of transport, through the example of the tramway in two main agglomerations : Montpellier and Strasbourg. The reticular dimension of these town-planning projects is analysed with reference to the ways a plurality of actors imagine, project and realise the construction of cities, rather than through a sectoral point of view only dealing with transport. This relational approach aims to connect the often divided dimensions of urban development and environmental attention that can be considered as the two faces of a same issue, i.e. the implementation of urban sustainability. The concrete translations of “green cities” imply several social transactions leading to practical compromises, the projects of tramways make particularly visible.