Due to their complexity and high social impact, urban infrastructure projects often face challenges in managing the design decision-making processes across disparate disciplinary and knowledge domain boundaries. This paper introduces the notion of design boundary dynamics to describe the various cross-boundary coordination phenomena associated with organising the design of infrastructure projects. Taking a practicebased theoretical stance, the paper presents findings of qualitative research on the nature and genesis of design boundaries and their relation to the strategic decision-making on a transportation infrastructure project. Findings illustrate the entangled processes, through which the disciplinary, knowledge-domain and stage-based design boundaries emerged as a result of unfolding project practices. Paper identifies the key role of resource allocation constraints, path dependency of project decisions, and problem-solving nature of design and concludes with strategic recommendations for upstream operational integration to mitigate the impact of design boundary dynamics on infrastructure projects.