Abstract: 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).