Many view nationalist ideologies and national identities across Europe as the outgrowth of economic, social, and political modernization in the 19th and early 20th centuries. Yet, there are few spatio-temporally disaggregated continent-wide tests of the relationship between modernization processes and nationalism. More importantly, it is theoretically unclear whether modernization led to national cohesion and stabilized Europe’s multi-ethnic states or whether it destabilized them by fueling non-state nationalisms and separatist mobilization. In this paper, we use the gradual expansion of the European railway network 1816-1945 to investigate how this key technological driver of modernization affected ethnic separatism. Combining new historical data on ethnic settlement areas, conflict, and railway construction, we test how railroads affected separatist conflict and successful secession as well as independence claims among peripheral ethnic groups. Difference-in-differences, event study, and instrumental variable models show that, on average, railway-based modernization increased separatist mobilization and secession. Exploring causal mechanisms, we show how railway networks can either facilitate mobilization by increasing the internal connectivity of ethnic regions or hamper it by boosting national market integration and state reach. In line with our theoretical framework of center-periphery bargaining, separatist responses to railway access concentrate in countries with small core groups, weak state capacity, and low levels of economic development as well as in large ethnic minority regions. Overall, our findings call for a more nuanced understanding of the effects of European modernization on nation building. Paper available upon request.