Megaprojects are increasingly common across countries and attract substantial political attention from a variety of actors. Recent studies have highlighted the need to move from an understanding of megaprojects as linear and rational processes towards a more nuanced approach that accounts for non-linear and conflictual aspects. Participatory governance is often proposed as a valuable resource in this regard. In this paper, we investigate the setting and design of two participatory venues operating in the context of the implementation of the Lyon-Turin high-speed railway megaproject: the Italian Observatory for the Turin-Lyon Railway and the French Public Inquiry. Empirical evidence shows that the Italian case featured substantial structural barriers to effective democratic participation. As for the French case, while better designed and implanted in its context, it featured important agentic limitations that undermined its democratic potential. On the basis of our case study, we therefore argue that both the Observatory for the Turin-Lyon Railway and Public Inquiry failed to promote democratic participation. We thus propose a deliberative approach to (the study of) of megaprojects. Whereas deliberative democratic ideas command growing interest across disciplines, these have found only limited application in the study of megaprojects. We contend that a deliberative democratic approach holds promise to improve the democratic and epistemic qualities of decision making on megaprojects.