This chapter investigates how rationalities pertaining to the governance of urban systems are often shaped by the knowledge assemblages that planners make use of in order to translate the complexities and uncertainties of urban reality into comprehensible and governable planning obejcts. More specifically, the chapter focuses on how the transformation of established knowledge assemblages may introduce new political rationalities into urban system governance. This is illustrated and discussed through four cases illustrating how established ways of knowing the transport system in Copenhagen have been challenged or transformed. We illustrate how new ways of knowing have contributed to shape the governance of the transport system in Copenhagen since World War II. Our cases suggest that the development of devices that are capable of institutionalising new ways of knowing may be a central political ingredient in reconstructing or transforming political rationalities that inform the governance of urban systems.