A set of points near the site of the Cumbria train crash site was faulty, an initial report has found.
Investigators found one of three stretcher bars was not in position, two were fractured and bolts were missing.
The bars join the moving rails, keeping them a set distance apart.
Network Rail chief executive John Armitt said his organisation was "devastated" by the report and he offered an "unreserved apology".
Virgin boss Sir Richard Branson said he "took his hat off" to Network Rail for being "dignified" in accepting responsibility for the accident.
The London to Glasgow Virgin Pendolino train derailed on Friday evening at Grayrigg, killing Margaret Masson, 84, and seriously injuring eight others.
The Rail Accident Investigation Branch (RAIB) said in its report that there was "no complete stretcher bar in place between the switch rails immediately before the derailment".
One of the stretcher bars was possibly fractured before the crash and one possibly after, it said.
It also said there was no evidence to indicate the driving of the train or the condition of the train were contributory factors.
However, it added that there was evidence that the last scheduled inspection of the points, known as Lambrigg 2B, on 18 February did not take place.
The report concluded that "the immediate cause of the accident was the condition of the stretcher bar arrangement at points 2B at Lambrigg".
Mr Armitt said: "Network Rail is devastated to conclude that the condition of the set of points at Grayrigg caused this terrible accident."
He added: "We would like to apologise to all the people affected by the failure of the infrastructure."
He told the BBC he would not resign over the accident because it was not the time to "abdicate his responsibilities".
Sir Richard added: "It is not for us to apportion blame but rather to work closely together as train operating companies with all our partners in the industry, particularly Network Rail, to ensure that this never happens again."
He said a solution would be to organise routine maintenance locally, rather than centrally, to ensure local managers took pride in ensuring the safety of train tracks.
Iain Black, 46, the train driver, who suffered a broken collar bone and a broken bone in his neck, has paid tribute to the emergency services who helped in the aftermath of the crash.
Mr Black, who remains in a serious but stable condition, said he was "distraught" that somebody had died.
The grandchildren of the crash's elderly victim have spoken of their devastation at losing a "generous, loving lady".
Her daughter and son-in-law Margaret Langley, 61, and Richard Langley, 63, were also injured in the crash, and are in a serious but stable condition in hospital.
Two other passengers also remain stable.
Rail safety expert Peter Rayner told the BBC's World Tonight the Cumbria derailment looked like a re-run of the 2002 Potters Bar crash, in which seven people died when a train derailed following a points failure.
"My instinct tells me that this is a situation where the system that Network Rail had put in place has not worked and these points have not been subjected to the inspection that they should have done," he said.
Deputy Chief Constable Andy Trotter from the British Transport Police said he was not ruling out criminal charges being brought.
"Witnesses are being interviewed. And in due course, consideration may be given to submitting a report to the Crown Prosecution Service," he said.
Transport Secretary Douglas Alexander told the House of Commons that investigators would continue to study the "immediate and underlying" reasons why three stretcher bars were damaged or missing.
Any recommendations for urgent safety improvements during the investigation into the derailment would be acted upon immediately, he pledged.
But the final report would take "some months to prepare", he added.
Work on removing the train carriages from the site is not expected to begin until the end of the week.
Contractors are currently building two temporary roads from steel across muddy fields to allow heavy lifting gear access, and cranes will take about 48 hours to set up.
A forensic examination carried out by the British Transport Police, the RAIB and Her Majesty's Railway Inspectorate is expected to continue at the site for several days.
Cumbria Police helpline: 0800 056 0146
Police family liaison centre: 0800 40 50 40
National Rail Enquiries: 08457 48 49 50
Meanwhile there have been calls for an independent public inquiry into the crash.
Louise Christian, the solicitor who represented victims of the Potters Bar crash, said not enough had been done since then to improve rail safety.
The Transport Salaried Staffs Association union supported the calls.
While the crash investigation continues, buses are operating from Preston to Carlisle and from Lancaster to Carlisle serving Oxenholme and Penrith, in both directions.
GNER, TransPennine, Northern and Virgin CrossCountry have said they are accepting Virgin West Coast tickets until further notice.