What are the impacts of a re-designed public transportation system in a city where social differences are heavily manifested in spatial separation, i.e. where exist certain inequalities concerning the access to daily activities and participation in urban life? Are the changes just of transport-related manner, resulting in changes in accessibility conditions? Or might there be further impacts on people’s mobility behaviour, or even on economic and spatial structures within the city, affecting the manifestation or dissolution of social differences? What, moreover, happens, if the new system presents a set of functioning problems, and the systematic changes are very drastic, neglecting existent peoples’ preferences and habits concerning the entire urban transport system, including private modes? What are the concrete reasons for people’s lack of capability to cope with the new system? Finally, are there any measures to be implemented to support the societal acceptance of the new transportation system, in favour of a profitable exploitation, social interaction among users and thus, social cohesion? These are the main interests of the PhD-related research presented in this article, giving some insights into the theoretical framework, the methodological approach and first findings concerning the “role of public urban transport in the context of social inclusion and cohesion”. The PhD has recently been started at the EPFL in Lausanne/Switzerland and is principally based on the case study area of Santiago de Chile. There the national government implemented in February 2007 the new Transantiago system, representing an ambitious approach to substitute the previous rather de-regulated low-quality service by a modern and efficient one. So a new trunk and feeder line network was re-designed from scratch, whose main skeleton is constituted by the existent public metro service and complemented by privately operated bus lines. Unfortunately, the first experiences with the Transantiago have been merely negative, due to a set of technical and institutional problems and further social and cultural obstacles. Even if the national authorities committed to implement all design modifications as soon as possible, some problems are expected to remain also in a long term. Today, the Transantiago presents a hot topic in political debates and everyone’s daily life discussions. After the subsequent theoretical insights, the case study area of Santiago de Chile is presented, describing the urban development framework, the previous transportation service and the main problems related to the new Transantiago system. On this basis the research hypothesizes and objectives as well as the methodological approach are introduced. The final part offers some first findings for the Santiago case and bridges to the conference topic dealing with spatial inequalities due to public action.