The present invention relates to a method for identifying and locating a defect in a train wheel or an axle of a train as the train moves along a rail track, the method comprising the steps of: sampling magnetic field measurements obtained from a plurality of magnetometer sensors so arranged to be in proximity to a moving wheel or an axle, the sensors arranged to span at least a length sufficient to obtain the magnetic field measurements from the entire wheel or the axle during one complete revolution of the wheel or the axle; and identifying and locating a defect in the wheel or the axle when a sampled magnetic field measurement is equal to or exceeds a threshold value.
