A sensorized axlebox-bearing unit for supporting a railway axle, comprising an axlebox adapted to be fixed in a stationary position; a rolling bearing carried within the axlebox and comprising an inner ring adapted to receive, in use, the railway axle to make it rotatable with respect to the axlebox about a common symmetry axis; a sensor unit comprising a sensor module integrally mounted to the axlebox and close to the inner ring of the bearing, and provided with a plurality of transmission elements for monitoring the axle and with a connection plug to be connected to an electronic data processing unit; respective sealing assemblies arranged to protect the rolling bearing; and a front cover and a rear cover to close opposite first and second frontal circular inlet openings of the axlebox; characterized in that, in combination: i)- the rear cover is made in the shape of two independent ring stretches (20; 21) integrally fixed, by means of respective screws (22, 23), to the first rear inlet opening of the axlebox, facing in use an "inboard" side of the axle; the two ring stretches (20, 21) being adapted to couple, in use, with a radially outer cylindrical lateral surface of the axle; ii)- a first ring stretch being defined by a semicircular profile flange arranged to close a first circumferential portion of the rear inlet opening of the axlebox; iii)- the second ring stretch being defined by a semicircular profile box shaped as a polygonal radial section toroid delimited by a radially outer cylindrical lateral wall which is coaxial with the symmetry axis, by opposite first and second frontal walls having a semicircular profile, which are arranged substantially perpendicular to the symmetry axis, and by a pair of flat radial walls perpendicular to the frontal walls (29, 30) and adapted to couple in contact with respective radial end edges of the first ring stretch, so that the box is internally hollow and has, toward the symmetry axis, a circumferential opening facing the symmetry axis; the second ring stretch being integrally fixed to the rear inlet opening of the axlebox independently of the first ring stretch; iv)- the sensor module being integrally carried, radially on the inside, by the radially outer cylindrical lateral wall of the box, pre-assembled within the box in a position facing the circumferential opening, while the connection plug is integrally carried, radially on the outside, by the radially outer cylindrical lateral wall of the box and is connected to the sensor module by means of an electric cable arranged within the box and against the radially outer cylindrical lateral wall of the box. i)- the rear cover is made in the shape of two independent ring stretches (20; 21) integrally fixed, by means of respective screws (22, 23), to the first rear inlet opening of the axlebox, facing in use an "inboard" side of the axle; the two ring stretches (20, 21) being adapted to couple, in use, with a radially outer cylindrical lateral surface of the axle; ii)- a first ring stretch being defined by a semicircular profile flange arranged to close a first circumferential portion of the rear inlet opening of the axlebox; iii)- the second ring stretch being defined by a semicircular profile box shaped as a polygonal radial section toroid delimited by a radially outer cylindrical lateral wall which is coaxial with the symmetry axis, by opposite first and second frontal walls having a semicircular profile, which are arranged substantially perpendicular to the symmetry axis, and by a pair of flat radial walls perpendicular to the frontal walls (29, 30) and adapted to couple in contact with respective radial end edges of the first ring stretch, so that the box is internally hollow and has, toward the symmetry axis, a circumferential opening facing the symmetry axis; the second ring stretch being integrally fixed to the rear inlet opening of the axlebox independently of the first ring stretch; iv)- the sensor module being integrally carried, radially on the inside, by the radially outer cylindrical lateral wall of the box, pre-assembled within the box in a position facing the circumferential opening, while the connection plug is integrally carried, radially on the outside, by the radially outer cylindrical lateral wall of the box and is connected to the sensor module by means of an electric cable arranged within the box and against the radially outer cylindrical lateral wall of the box.