A system (10) and associated method for detecting a material discontinuity in a magnetisable article (12) has a sensor unit (16) which includes a magnet 18 and at least one magnetic field coupling sensor S. The magnet (18) is supported a distance above the rail (12) so that the lines of magnetic flux (22) loop through the rail (12). The magnetic field of the magnet (18) causes the surface (14) of the rail (12) directly below the magnet (18) to become polarised opposite to the facing pole of the magnet (18) and the regions distant from the magnet (18) to become inversely polarised. The coupling sensor S is placed and held at a fixed position relative to and in the active magnetic field of the magnet (18). The coupling sensor S measures the flux coupling between the rail (12) and the magnet (18).
