Throughout the nineteenth century, powerful railway unions in the USA and the UK cultivated an expansive system of voluntary sickness, death, unemployment, and superannuation benefits. By the early twentieth century, the movements had diverged: while the British Amalgamated Society of Railway Servants relinquished its commitment to voluntarism in favor of state healthcare and pensions, the American Railway Brotherhoods persisted along voluntarist lines, resisting social insurance in favor of exclusive schemes for their white male membership. What accounts for these diverging orientations? I highlight the importance of organizational forms as a lens for understanding comparative trade union strategy, emphasizing the role of law in designating legitimate forms of working-class association. I demonstrate that governing elites in both countries promoted voluntarism as a benign form of working-class organization throughout much of the nineteenth century. Consequently, I argue, early American and British trade unions adopted benefits in part because they enabled them to mimic the far more respected and legitimate friendly and fraternal mutual benefit societies. Toward the end of the century, the context had changed: while alternative organizational avenues were opened for trade unions in the UK, benefits presented an ongoing organizational lifeline for American unions. In defining and redefining the boundaries of legitimate forms of workers’ associations, legal decisions in both countries shaped not only trade union organizing strategies in the short run but also their positioning in broader social struggles.