A sheath for railway track fixings, and a procedure and tools for replacing the sheath in a sleeper, the sheath being free of projections that prevent the rotation thereof and comprises a helical rabetting on an outer surface configured to house a helical projection of the sleeper and thread with respect to the helical projection, the sheath comprising an inner thread configured to house a male thread of a coach screw, wherein the helical rabetting comprises an upper part having a concave curved configuration and a lower part having a straight configuration, the sheath further comprising removable anti-rotation means to prevent a rotation movement of the sheath when it is housed in the sleeper.