Modern metropolitan areas involve large concentrations of economic activity and the transport of millions of people each day between their residence and workplace. We use the revolution in transport technology from the invention of steam railways, newly-constructed spatially-disaggregated data for London from 1801-1921, and a quantitative urban model to provide evidence on the role of these commuting ows in supporting such concentrations of economic activity. Steam railways dramatically reduced travel times and permitted the rst large-scale separation of workplace and residence. We show that our model is able to account for the observed changes in the organization of economic activity, both qualitatively and quantitatively. In counterfactuals, we nd that removing the entire railway network reduces the population and the value of land and buildings in Greater London by 20 percent or more, and brings down commuting into the City of London from more than 370,000 to less than 60,000 workers.