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.
