The socio-technical dynamics of developing new urban transport systems are explored. Based on the analysis of empirical material from the study of the Transmilenio in Bogota and the Metro in Copenhagen, it is proposed that the design, construction and operation of urban transport systems constitutes a process where the actors involved negotiate and actively distribute agency in the components of the new system. The character and outcome of this process play a role in the stabilization of the system over time. Additionally, this process takes place in a setting dominated by established actors: institutions, technologies and interest groups. This setting is analysed as an arena of development, a concept that provides a framework to account for the interaction of existing and new systems. This approach is based on theoretical developments from Science and Technology Studies, especially Actor-Network Theory and Large Technological Systems, and contributes to the current research on the dynamics of change and permanence in built environments.