France is finally going for it: the SERM (express regional and metropolitan services) are to be launched and radically improve mobility possibilities around at least ten French metropolises outside of Paris. By providing a frequent, simple train service with an extended time-range, public authorities want to provoke a massive modal shift to trains, with the overarching goal of reducing the environmental impact of mobility. This change comes with its range of challenges: how to prepare small and deserted stations for a massive ridership increase, how to deal with intensifying flows in already crowded central stations? how to maximise the impact of modal shift, without causing induced automobile traffic? how to dimension amenities and services to enable a radical change in mobility chains? how to avoid reinforcing urban sprawl? In this presentation, we will mobilise our experience of working with these projects, using both research and consulting tools. Working on the Bordeaux, Toulouse and Rouen metropolises projects, we developed several methods to collect precise existing data, assess the needs of the current and future stations, co-construct the park-and-ride amenities… Our approach aims to optimise the carbon reduction impact of the SERM projects, using mobility models, territorial analysis and a global environmental perspective. We will show that the modal shift challenges are still largely open – still, our regional-scale methods may help to optimise this opportunity for a radical change in the French metropolitan mobility system.