This research is about the relation of the unionism and the process of consolidation of a democratic society in Brazil. In particular, it studies the limits and possibilities of the joint action of the unions with other social movements, with the objective to extend its base of political support for the defense of the workers´ interests and the rights of the work, in general. It is also a reflection about the union action in a context of transformation in the world of the work and restriction of the public space and class actions, having as focus the current Brazilian society, where the correlation of forces was altered with the arrival of the Workers´ Party (Partido dos Trabalhadores - PT) to the Presidency of the Republic. To make such analysis possible, a study was carried through on the Union of the Railroad Workers of the Zone of the Central of Brazil, headquartered in Rio de Janeiro. This union, since the decade of 1990, is facing the consequences of a process of grant of concession of the railroad transport public services as part of the National Program of Privatizations that had beginning in the Collor government and was accomplished in the FHC government. The study approached these questions, as well as the strategies of syndical action adopted by the union, with emphasis in the joint action with other social movements. The privatization of the railroad transport services in Brazil demarcates a new phase for workers with a tradition in the syndical struggle. Theses workers faced in the decade of 1970 the trashing of the RFFSA and its extinguishing in 2007, after the concession of its services to private enterprises.