Video images of the faulty track in the Cumbria derailment were taken by an inspection train two days before the crash but not viewed at the time.
Inspectors have said faults with the points led to the crash
Network Rail's measurement train passed at high speed through Grayrigg on the West Coast Main Line on 21 February.
The train's purpose is to alert the firm to defects only in the rails so it did not flag up the problems with the points being blamed for the crash.
A spokesman said the film was used as a "backwards reference tool".
One woman died and eight people were seriously injured when the Virgin Pendolino train derailed.
Two new roads, built to provide access to the crash wreckage, are due to be completed.
Emergency services say the first of the massive cranes, which will be used to remove carriages from the site could start work on Thursday.
Network Rail is hoping the line will be operational again after 10 March. Bus services are currently replacing trains in the area.
Regulators, meanwhile, have said they were worried that not enough was being spent on the national rail infrastructure.
The Office of Rail Regulation also said the number of points failures increased slightly in the lead-up to the derailment in Cumbria.
The initial report from the Rail Accident Investigation Branch said faults with the points meant the tilting train could not follow its intended path over the tracks and derailed.
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.
The inspection train's equipment can automatically alert technicians on board to defects in the condition of the track's metal but would not pick up on missing stretcher bars.
It travels too fast to allow anyone to spot such faults at the same time through the video camera.
The Network Rail spokesman confirmed that the inspection train's video recording showed one of the stretcher bars and some of the bolts were missing.
But he said the video is only viewed after an incident.
The train runs at up to 125 mph and any such problems could not be picked up by watching it in "real time", he added.
The 18 hours of video footage needs to be slowed down for images to be seen accurately.
"It would probably take someone most of the month to watch one day's-worth of data," he said.
Network Rail's maintenance policy requires visual checks of the West Coast Main Line to take place every week.
However, in its initial report, the RAIB said there was evidence that a scheduled visual inspection of the track on 18 February had not gone ahead.
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.