4. Rail defect detection and monitoring techniques

Manual inspection method is still widely used in most routine track inspections until today since it can directly figure out rail defects. However, it needs experienced workers and involves significant human input and judgement [48]. Therefore, NDT&E techniques, which enable rail inspection in an automated manner, are in need. NDT techniques were advocated for rail inspection as early as the 1920s [49]. Ultrasonic testing (UT) emerged in the 1960s became dominant in rail inspection [10, 50]. With the development of UT, European countries and Japan have released a variety of forms of ultrasonic rail flaw detection equipment, such as portable types, hand-push types, road and rail dual-use vehicles and specialised railtesting trains [28]. While being extensively utilised, both the magnetic induction testing and conventional UT methods are not suitable for all defect scenarios; for

example, they offer poor sensitivity to defects located in the rail web and rail foot [51]. A wide variety of inspection techniques are under research and development with the target to enhance the detection capability.
