Turning Passenger Trains Into Rail-Crack Detectors

Turning passenger trains into rail-crack detectors is possible with a new ultrasonic device developed by physicists at the University of Warwick in England (Steve Dixon, s.m.Dixon@warwick.ac.uk). Current ultrasonic track-inspection equipment must be operated on special work trains running 20-30 miles per hour. With the new device, the idea is to enable an ordinary fleet of passenger-carrying trains, traveling as fast as 200 miles per hour, to continuously and routinely check for early signs of track failure.

The new ultrasonic technique can detect track defects within 15 mm of the rail surface. Furthermore, it can detect "gauge-corner" cracks, those that occur from rolling wheels making contact with the inside of a rail head (the wide stubby top part of a rail). Track failure from gauge-corner cracking is believed responsible for numerous accidents, including a UK train derailment in October 2000 that killed four people.

Mounted on a train, the device generates "low-frequency, wide-band Rayleigh waves," multiple-frequency sound waves that travel swiftly along the length of the surface skin of the rail. Different frequencies penetrate to different depths in the rail, with the lower frequencies having a deeper penetration of around 15 mm. If the waves encounter a crack, they get partially blocked or reflected in a way that can be detected by the device, which can then record its exact location and depth, by determining which frequencies are able to pass underneath the crack.

Preliminary results suggest that this technique can even detect changes in microscopic structure and stress levels within the rail that could identify crack-susceptible stretches of track. However, more testing is necessary to confirm this capability, and further development is required to bring the device from the lab to real-world passenger trains. The work, published in the June 2004 issue of INSIGHT, the Journal of the British Institute of Non-Destructive Testing, was presented at this week's 7th International Railway Engineering conference in London. (University of Warwick press release, 5 July.)