A blocking end cap for a rolling bearing in a railway axle box of a supporting device for a railway axle is provided. The cap is shaped as a concave disc delimited by an annular side wall having a symmetry axis (A), a bottom wall arranged transversely to the symmetry axis (A), and an elbow-shaped, annular joining portion, connecting the bottom wall to the side wall. The thickness of the bottom wall measured in a direction parallel to the symmetry axis (A) is continuously variable in the radial direction between a maximum (S1) situated at the symmetry axis (A) and a minimum (S2) arranged at a radially outer edge of the cap defined by the side wall.