The end of the Modern era represents the beginning of a new phase for city development that requires urban planners, policy makers and civil servants to experiment with open decision-making processes, contributions from disciplines beyond urbanism to face the increasing complexity of society. Although in recent years many Western cities become laboratories open to innovation and contamination, very few studies focus on analyzing participatory processes in large urban transformation and the management of their legacy. The contribution presents a detailed and reflective account of SUPERTRENTO – Participatory Urban Scenarios for Ecology and Regeneration, an experimental participatory urban planning process initiated by the Municipality of Trento in collaboration with the collective Campomarzio. The focus lies on describing the practice-led, research-through-design methodology developed to engage citizens in the urban transformation of a major railway area slated for undergrounding. Combining results with existing literature, it draws conclusions on how participatory urban planning becomes spaces to learn and exercise democracy and concludes on the relevance of research through design ways to institutionalize the legacy of these practices to serve as mines of knowledge on the city and places to improve urban policy design.