Railway brake system for a railway vehicle and braking method of a railway vehicle comprising such a system

The invention relates to a railway braking system comprising a service brake (6) having a movable piston (8) delimiting with a body (2) of the system a service brake chamber (13) supplied by a first source, and a parking brake (7) having a locking device (20) admitting a position where it immobilizes said piston in a position, and a control device (23, 24) movable delimiting with said body a parking brake chamber (25) supplied by a second source and admitting a stable position where it maintains said locking device in its position; with, when said control device is in its stable position, said service chamber being supplied by said second source whose pressure is higher than that of said first source in order to apply a braking force greater than the force applied when said piston is in its position.