Finally, Chapter 9 uses 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. These effects concentrate in countries with small groups in power, weak state capacity, and low levels of economic development, as well as in large ethnic minority regions. Exploring causal mechanisms, we show how railway networks can facilitate mobilization by increasing the internal connectivity of ethnic regions or hamper it by boosting state reach. Overall, our findings call for a more nuanced understanding of the effects of European modernization on nation building.