Network Rail (NR) is to be prosecuted over the 2007 Grayrigg train crash in Cumbria in which one passenger died, it was announced today.
The Office of Rail Regulation (ORR) said it had started criminal proceedings against NR for a breach of health and safety law which caused a Virgin Trains Pendolino train to derail near Grayrigg on February 23 2007.
Passenger Margaret Masson was killed and 86 people were injured, 28 seriously.
Earlier investigations as well as last year's inquest into the death of Mrs Masson concluded that the derailment was caused by a poorly maintained set of points.
NR is facing a charge under section 3(1) of the 1974 Health and Safety at Work Act.
The ORR said: "This results from the company's failure to provide and implement suitable and sufficient standards, procedures, guidance, training, tools and resources for the inspection and maintenance of fixed stretcher bar points."
The stretcher bars hold the moveable rails a set distance apart when the points are operated.
The train that derailed was the 5.15pm London to Glasgow Central service.
The ORR railway safety director Ian Prosser said: "We have conducted a thorough investigation into whether criminal proceedings should be brought in relation to this derailment which caused the death of Mrs Masson and injured 86 people.
"Following the coroner's inquest into the death of Mrs Masson, I have concluded that there is enough evidence, and that it is in the public interest, to bring criminal proceedings against NR for a serious breach of health and safety law which led to the train derailment."
Mr Prosser went on: "My thoughts are with the family of Mrs Masson and those injured in this incident. The ORR will do everything it can to ensure that the prosecution proceeds as quickly as possible.
"The railway today is as safe as it has ever been but there can be no room for complacency. The entire rail industry must continue to strive for improvements to ensure that public safety is never put at risk."
The first hearing is due to take place at Lancaster Magistrates' Court on February 24.
NR network operations managing director Robin Gisby said: "The Grayrigg derailment in 2007, resulting in the tragic death of Mrs Margaret Masson, was a terrible event.
"Network Rail has not hidden from its responsibilities. The company accepted quickly that it was a fault with the infrastructure that caused the accident. We again apologise to Mrs Masson's family.
"Since the derailment, we have worked closely with the authorities, conducted comprehensive and detailed investigations and made substantial changes to our maintenance regime.
"Today there is no safer form of travel than rail and it is important that the rail industry seeks ways to make it safer still."
The case is almost certain to be committed to a Crown Court.
The maximum penalty a magistrates' court can impose for the offence is a fine of £20,000.
Last year, NR was fined £3 million at St Albans Crown Court after admitting safety breaches involving a set of points which led to a derailment at Potters Bar in Hertfordshire in May 2002. Seven people were killed.
NR had assumed responsibility for the crash from its predecessor rail infrastructure company, Railtrack.
The ORR delayed any decision on bringing its own prosecution over Grayrigg until the inquest into Mrs Masson's death concluded in November 2011 and after the Crown Prosecution Service (CPS) had said it would not bringing a prosecution.
Manuel Cortes, leader of the TSSA rail union, said: "We welcome this prosecution, although we are disappointed that it has taken five years for this to happen."
Bob Crow, general secretary of the Rail, Maritime and Transport (RMT) union, said: "No organisation has fought harder than the RMT for the lessons of the Grayrigg disaster to be learnt and for the core issues that led to this preventable tragedy happening to be tackled so that we don't get a repeat in the future.
"This prosecution leaves the ORR facing both ways - on the one hand as the regulator taking this action, and on the other as the budget holder demanding yet more cuts from NR that add to the risk of having inadequate time and staff to complete proper patrols and maintenance."
He went on: "There remain systemic problems which have failed to be addressed since this derailment and they have been compounded by the ORR cuts regime and the looming threat to jobs of the (Sir Roy) McNulty rail review.
"This prosecution does not let the ORR off the hook for their role as the driver of the cuts that are still ongoing, and RMT repeats its call for an urgent public inquiry that establishes all the facts and which looks at the whole culture of penny-pinching and short-staffing that is prevalent on the railways."