The Brotherhood of Railroad Signalmen is a little-known technical and political organization that gained power during the opening decades of the twentieth century through the increasingly complex nature of members’ work, the vision of its leaders, and their abilities to gather support from other unions and the federal government. This thesis is organized around three themes: first, how the growing complexity of signal systems continually challenged signalmen to broaden signalmen’s skills, which, in turn, gave them an advantage in asking for recognition as a skilled craft union; second, how the skills that signalmen employed brought them into conflict with other unions over signal department jobs; and third, how, despite having only between 10,000 and 19,000 members, the organization’s leaders learned to negotiate using reason, evidence, and logic to demonstrate the union’s importance in the industry as the custodians of public safety and rail traffic efficiency.