Beginning in 1878 during the Amazon rubber boom, two North American investors bought the rights to construct the Madeira-Mamoré railroad to transport rubber from the Brazilian interior to eager North-Atlantic markets. Beginning in what would later become Porto Velho, Rondônia, more than twenty thousand workers laid nearly three hundred kilometers of tracks bisecting the Amazon rainforest from the Amazon River to the town of Guajará-Mirim at the Bolivian border. Most of the workers were Black women and men, either Afro-Brazilians from other regions searching for work just decades removed from the abolition of slavery or Caribbean migrants participating in trans-Caribbean networks of labor migration. My dissertation explores the centrality of Black migration to the northwestern Amazon region for the construction of not only the Madeira-Mamoré railroad, but the socially, culturally, and economically connected worlds of modern Brazil and the greater Caribbean. Their experiences challenge narratives of late imperial and republican Brazilian history; Black migration to the periphery and the dispossession of Indigenous and Quilombo land drove much of the economic and political development in the industrializing southeast. The work done by Black women and men near Porto Velho demands attention to gender and sexuality, both in emphasizing the downplayed contributions of Black women to the project, and in theorizing the development of modern capitalism in the region. Third, the migration and labor patterns on the Madeira-Mamoré parallel patterns seen in other transnational imperial worksites around the Caribbean. This study expands our understanding of networks of migration, labor, and culture that regularly connected communities in Florida, Panama, and the Antilles, positing that the limits of the greater Caribbean stretch farther south than conceived before.