He led the drive for one big union embracing all rail workers. In his early twenties, he had become an official of the Brotherhood of Locomotive Firemen, a bastion of crafts separatism. He started out as a champion of compromise and arbitration as avenues to harmonious and equitable labor-management relations. But experience convinced him that the robber barons in command of major railroads were less interested in peace than in pitting one brotherhood against another.

The collapse of a strike by rail switchmen in Buffalo in 1892 after the other crafts refused to come to their aid prompted Debs to resign his post with the firemen and become prime mover in setting up the American Railway Union, which cut across all craft lines and even took in longshoremen, car builders, and coal miners. The infant union had phenomenal growth, quickly eclipsing in size the combined membership of all the old-line brotherhoods. But its demise was equally swift.