When the first section of the Metropolitan Railway opened in January 1863 in London, debates in Parliament emphasized the need to conceive of railways as a system of interconnected circles instead of the lines and termini that had been built since the 1830s. Similar debates took place in Paris around this time, although no plan was implemented before the opening of the Métropolitain’s first line in 1900. The use of geometric terms such as rings, circuits and circles proliferated throughout the process, illustrating new ways of connecting the railways, and, more importantly, embryonic ideas about how the two cities could use transport technologies for shaping their own growth. Doing so was dependent on how, where and why the notion of the public benefit was articulated. Railways encapsulated both constraints and possibilities for the transformation, real and imagined, that the two metropolises were to experience.