Twenty years ago, on 17 October 2000, a train derailed just south of Hatfield station while travelling at around 115mph. Four people died, 70 were injured. The accident was caused by rolling-contact fatigue (metal fatigue) of the left-hand rail, which broke as the train passed over it.
Railtrack (the private infrastructure operator) and Balfour Beatty (the contractor responsible for track maintenance on that line) were found guilty of breaching health and safety laws. Railtrack was forced by the government into administration, and publicly owned Network Rail was established.
(Permalink) Balfour Beatty Hatfield crash Network Rail Railtrack