By Tom Symonds
Transport correspondent, BBC News
The inquiry into the Cumbria rail tragedy is likely to be long and sweeping.
Investigators are gathering evidence on the condition of the track, the positions of the carriages, the actions of the driver, the train's on-board data recorders, the testimony of maintenance workers, and piles of paperwork.
Investigators are focusing on a set of points
The investigation is being led by the newly created Rail Accident Investigation Branch of the Department for Transport, which is trying to find the causes of Friday's crash near Kendal.
British Transport Police are pursuing a parallel investigation aimed at discovering who was to blame and whether any charges should be laid.
The RAIB has already made clear it is focusing on the set of points just before the crash site.
At this location trains can switch from one of the two tracks to the other. It normally happens only when one line needs to be closed for some reason.
The investigation is examining the state of these points. Was the track itself defective? Were the points set correctly?
Already some have suggested the nuts, bolts and bars that hold the tracks at the correct distance apart may have come loose. This was the cause of the Potters Bar train crash in 2002, in which seven died.
Rail bosses have insisted the network is safe
Network Rail says the points this time are of a different type, but so far it is not clear if loose parts were to blame.
If there was a mechanical defect, the maintenance regime for the track will come under sharp scrutiny.
Network Rail says a monthly maintenance check of the points was carried out on 3 February. There should also have been at least one weekly visual check since then.
Every 13 weeks a major maintenance check takes place, and every year points are dismantled and rebuilt to ensure they are working correctly. Track workers are being interviewed and maintenance records examined.
Other factors may have played a part. Virgin Trains has suggested the embankment down which the train slid may have become sodden in the rain and given way under the weight of the derailed carriages.
Network Rail had been hoping changes to the way it maintains the tracks might prevent a crash like this happening.
After the spate of disasters in the late 90s, the Hatfield derailment in 2000 and Potters Bar, the contracting out of daily track repairs to private companies was ended. The work came in-house.
There is evidence that the reliability and safety of trains has improved since. But if Network Rail's basic procedures are found to be at fault, the company, and the rail industry could go through another uncomfortable period of criticism.