A cooling tank (1) for the thermal treatment of a head (20) of a rail, comprising a container (1') defining a longitudinal symmetry plane (X) and adapted to be filled with a cooling liquid wherein the head (20) to be thermally treated can be at least partly immersed parallel to the symmetry plane (X), said container (1') having a bottom (8) with a single row of holes (23), arranged at said symmetry plane (X), for generating cooling fluid flows in said container (1'), wherein the container (1') is equipped with a pair of first longitudinal bulkheads (13, 14) arranged symmetrically with respect to the symmetry plane (X), wherein said first longitudinal bulkheads (13, 14) have a respective upper concave-shaped stretch (6, 7) with respect to the symmetry plane (X), and wherein the upper stretches (6, 7) are separated from each other by a distance greater than the width of the rail head (20), so as to define an area of the container (1') adapted to house the rail head (20) at least partly.