A disc-brake unit (10) for vehicles on rails, comprising a brake pincer consisting of a pair of jaws (14), provided with relative friction gaskets (16), intended to rub on the two opposite surfaces of a disc (12). The jaws (14) are hinged at one of the ends of a pair of levers (18). The levers (18) are pivoted on respective hinges (20) fixedly connected to the main body of the braking unit (10), and are actuated by a braking force motor (22) provided with at least one first piston (26) that moves with alternate motion along an axis (A). The braking unit (10) also comprises a group (68) for automatically regulating the clearances between the friction gaskets (16) and the disc (12). The piston (26) transmits the braking force to a thrusting mechanism (48; 48′) inserted inside the braking force motor (22) to amplify and transfer the force, through the group (68) for automatically regulating the clearances and along an axis (B) perpendicular to the axis (A), to the ends of the levers (18) opposite the ends on which the jaws (14) are hinged.