The research question is framed in the state of the art of sustainable mobility and urban walkability guided by an integrated urban planning/infrastructure approach. The ongoing study has been developing in the Spoke Urban Mobility of the MOST National Centre for Sustainable Mobility within the collaboration between FS Italiane Group, University of Cagliari and University of Parma. This study starts from the awareness that active accessibility and intermodality of railway stations is a fundamental aspect to achieve sustainable and walkable cities. In fact, the impact of last mile mobility (15 min walkable area), in the use of public rail transport, is growing, and requires a specific attention and a holistic approach to the analysis and design of stations and their urban surroundings. The purpose of the study is the definition of the SIWI - Sustainable Intermodal Walkability Index - for the evaluation of urban spaces in contiguity with railway stations, with particular reference to the morphological, functional and environmental characteristics enabling pedestrian and cycle accessibility. The goal is to strengthen sustainable mobility, as a result of a combined use of different public and green modes of mobility. The Index is considered a possible assessment framework for sustainable design of stations, with the support of digital tools and guidelines for infrastructure design. In this framework, the paper describes purposes, methodology and first results from the case study, as well as the remarks on the opportunities, limits and future developments of the study.