GAUGE corner cracking, the form of rail metal fatigue believed to have been partly responsible for the Hatfield tragedy, has become an increasing problem on rail networks around the globe over the last five years.

It is caused by heavy trains running at speed around bends. The wear caused by train wheels as the heavy locomotives go around the bends can cause tiny surface cracks in the rail usually on the inside corner.

If caught early enough, the cracks can be filed away before they spread deeper a bit like filing a cracked finger nail before it splits. If not caught early, they grow and the entire rail has to be replaced.

Gauge corner cracking gets worse the tighter the curve, the higher the speed and the heavier the train. Engineers have been aware of it for less than ten years but it is now occurring on rail networks around the world, including the US, France, Germany and the UK. There has been a 'significant increase' in gauge corner cracking in the last five years. An international effort is under way to find out why.

"At the moment, nobody knows quite why it is happening," said Railtrack's regional director for the North East, Nicholas Pollard. The age of the track seems to make little difference: the cracking has been found in track less than a year old. Track maintenance engineers however know the locations where it is most likely to occur. Regular inspections involve a visual check followed by ultrasound scanning. If problems are discovered, temporary speed restrictions can be put in place until the section of rail has been repaired or replaced.