A counterweight is provided for tensioning the wire of a catenary in a railway line, comprising a metal shell (1) made up of a frame (1a) and a surrounding body (1b) which is filled with a filler material (2), which integrates at least high-density concrete in combination with a metal material as an additional filler for increasing the density of the counterweight. The frame (1a) is embedded into the filler material (2) and in turn, is linked to the surrounding body (1b) by a welded joint.