Cities in Latin America in particular have been investing in new transportation networks such as bicycle systems, metros and bus rapid transit (BRT) technologies in recent years. These infrastructures are promoted as cures for trenchant social and spatial divisions as much as for traffic gridlock and vehicular pollution. This article unpacks the theory that infrastructures might mend cities that have been fragmented into disparate parts by uneven capitalist development. I argue that this ‘infrastructural solidarity’ thesis relies on a troubled imagination—shared across urban design and strands of urban theory—that infrastructures are static, formal arrangements that concretize relations and enforce social cohesion or fragmentation. This article draws on qualitative research on the TransMilenio BRT system in Bogotá, Colombia, as well as on the work of Bruno Latour, suggesting that the political life of infrastructure is better revealed when such systems are understood as dense knots of shifting relations with complex temporalities. Arguments for the value of this type of actor-network theory (ANT) reading often skew towards the esoteric, but the TransMilenio case shows how sorting through infrastructural ontologies actually matters in terms of how urbanists—academic and practicing—conceive of and work towards just and functional cities.