This paper concerns the emergence and diffusion of radical innovations in the context of sustainability transitions. We confront the typical understanding in the Strategic Niche Management framework with an in-depth longitudinal case study of French modem tramways (1971-2016), which represents a particular technology class: local infrastructure systems. The case confirms the relevance of existing SNM-concepts, but also points to three pattern deviations: 1) incumbent actors from neighbouring regimes can play a leading role in the development of radical alternatives, 2) the early formulation of highly specific visions can effectively guide search paths (as opposed to a usual prescription about more open-ended approaches to foster innovative variety creation), and 3) particularly influential projects (which we call 'landmark projects') can decisively accelerate innovation developments. Exploring a greater variety of diffusion and transition patterns (based on temporal interactions of causal mechanisms and varying roles played by different actors) is a fruitful way forward for sustainability transitions research.