This paper explores the generative capacity of activist movements defending their community assets from commodification or closure to produce new forms of social infrastructure. We explore this through the case study of activism along the Westway, a motorway in West London. The area has a strong tradition of community activism, particularly in the 1960s and the 2010s. Through a Participatory Action Research approach, we elaborate a historical account of the formation of social infrastructure under the Westway and surrounding spaces. In doing so, we develop a framework—processes, alliances, and capacities—to understand how communities and activist groups contesting the loss of community spaces can generate new forms of social infrastructure—both physical spaces such as play spaces or cultural venues, and social and caring relationships such as friendships or support networks. These informal spaces and alliances can shed light on how to build alternative, bottom‐up forms of social infrastructure.