Wealth from western investments lit up the Gilded Age. East and West, it financed the mansions, balls and philanthropy that were integral to upper-class culture. Historians of capitalism have argued that a national upper class coalesced during the late nineteenth century and that the development of a common culture was essential to its formation. Much of this work has focused on the Northeast. How did this play out in the Trans-Mississippi West? This article explores the roles that architects and the buildings they designed played in the intertwined processes of class formation, capitalist expansion and the advancement of white settler colonialism in the American West. It begins in the early 1880s, when Henry Villard (1835–1900), president of the Northern Pacific Railway, launched an ambitious plan to complete the transcontinental railroad and enlisted the architects McKim, Mead & White and their assistant, Cass Gilbert (1859–1934), to design buildings of all kinds along the line — an unprecedented move for a new western railroad. It then follows Gilbert back to St Paul to examine two major projects, one for local clients and one for Villard’s colleague, the eastern capitalist William Endicott, Jr (1826–1914). As agents for eastern capitalists and their counterparts in the West, the architects and the buildings they designed activated in the West an elite aesthetic and professional culture initially generated in the Northeast. Operating across local, regional and national scales, they contributed to the expansion of capitalist markets, the formation of a national upper class and, more broadly, the processes of settler colonialism in a rapidly changing region.