An initial report by the Rail Accident Investigation Branch has found that a set of points near the site of the Cumbria train crash was faulty.
Investigators discovered one of three stretcher bars was not in position, one had nuts and bolts missing and two were fractured.
The bars join the moving rails, keeping them a set distance apart.
The London to Glasgow Virgin Pendolino train derailed on Friday evening at Grayrigg, near Kendal, killing an 84-year-old woman and seriously injuring eight others.
Here are the answers to some of the key questions following the report.
How devastating are the findings of the report?
The Rail Accident Investigation Branch, a relatively new body, had been expected to do little more than confirm the basic facts of the crash.
But it has reached firm early conclusions about the crucial role of the points.
The apparently appalling state of this cross-over section of the line seems to have been plain for the investigators to see.
Their report is factual, and does not at this stage apportion blame, but it is far worse than Network Rail's bosses could have feared.
What happens now?
This is just the beginning of the investigation.
The interim report has answered the question, how did this happen? Now the investigators will want to know why it happened.
Network Rail boss John Armitt will face difficult questions
They will be identifying track workers and their managers who may have had contact with the points.
Some will no doubt be interviewed under caution in the presence of the British Transport Police.
Network Rail keeps detailed records of its work, and these will be seized.
One important piece of evidence will be video recordings from the New Maintenance Train, a high-tech train which scans the rails using ultrasound and makes a video recording.
This ran through the points last Wednesday, and any damage should have been picked up.
Gathering the evidence could take months.
How does the cause of the crash compare with Potters Bar?
It is remarkably similar.
The derailment of the train at Potters Bar station was caused by a faulty set of points.
Like Cumbria, there were loose nuts on the track. A stretcher bar, which holds the moving part of the points together was broken.
Maintenance records were seized. But the crash at Potters Bar demonstrates the difficulty of investigating incidents like this.
In that case, police were never able to answer the question, why did it happen? They simply could not trace the track workers responsible, and close the case.
How much pressure is Network Rail now under?
This is turning into the company's biggest crisis since it was created a few years ago.
Until now, Network Rail was enjoying something of a honeymoon period.
Safety and train punctuality were improving, billions of pounds were being invested in the railways, and the government was in full support of the strategy.
On Monday one railway executive said "the honeymoon's been going on longer than it should".
The company is now facing demands from train operators that it radically improves the maintenance of points and other track.
Network Rail had already taken track maintenance 'in house' - it was no longer contracted out.
But that now means Network Rail stands alone as difficult questions are asked about its competence.