One of the causes of train derailment
As we all know, track gauge is a measure of the width between the two parallel railway tracks. (By the way, US Standard Railroad Gauge Urban Legend suggests an interesting historical reason for the standard gauges used today). With age, unmaintained tracks tend to move further apart. This increased distance is known as track gauge spread.
The standard track gauge in the UK, (and in many other countries), is 1435mm ± 30mm. i.e. trains can safely run on a track with a gauge spread tolerance of 30mm.
In late November 2000, a London to Glasgow train derailed at just 15 mph near Motherwell (Scotland). The Health and Safety Executive ruled that the derailment was caused by a track gauge spread of 60mm (twice the tolerated amount).
At the time, this (comparatively minor; only 4 people injured) accident was one more thing that Railtrack (the private company responsible for Britain's rail infrastructure) had to worry about. It came 3 weeks after a major derailment at Hatfield near London, which was caused by a different type of rail problem: corner gauge cracking. Suddenly the full scale witch-hunt for damaged rails had to check for another problem: the affected track at Motherwell had been tested for damage only the previous week, but the engineers overlooked track gauge spread.