My dissertation asks the question: what, if any, was the relationship between human mobility and state formation in Argentina and Chile during the late nineteenth century? To answer this question, I examine the development of the Transandean Railroad, built across the Andes to connect western Argentina and Chile. While state makers and capitalists designed the project to strengthen connections between the two countries, by the time the project finished in 1910 the territorial division between Argentina and Chile had grown more stark than it had been at the time of the railroad's conceptualization in the 1850s. To address this development, I examine how the project shaped and was shaped by the environmental and technological relationships that allowed people to move across the Andes and constituted a transandean space and community. I hypothesize that the strengthening of Argentine and Chilean state territories in the central Andes occurred in part because the railroad project altered environmental and technological relationships of mobility central to the transandean community, thereby positioning the state as an important force for the reconstitution of stable material relationships. To test my hypothesis I analyze the interactions between the project and the transandean community and trace out the ways that non-state actors reinterpreted their material world, asking what role they gave the state in dealing with a changing transandean space. I organize my study through two phases of the Transandean Railroad: its conceptualization (1850s to 1880s) and its construction (1880s to 1910). For the first phase, I aim to answer questions about why the Argentine and Chilean states supported the project and how knowledge was produced for planning the railroad. For the second phase, I reconstruct how local communities, railroad workers, engineers, and muleteers interacted with the construction project, and how these interactions did or did not influence state formation.