The Rail Accident Investigations Branch (RAIB) said the train was travelling around 73mph – 2mph below the area’s speed limit – when it collided with stones washed out onto the track, raising questions about the drainage system in the area.
Driver Brett McCullough, 45, conductor Donald Dinnie, 58, and passenger Christopher Stuchbury, 62, were killed in last August’s derailment, which followed a period of unusually heavy rainfall.
An interim report by the RAIB into the tragedy said there was no evidence of track defects or any fault with the Glasgow-bound train.
However, it said there was no evidence that a drain upslope of a catchpit – situated on a steep, gorse-covered slope – had been inspected between its construction in 2012 and the time of the accident.
The slope in question had a "history of landslips and rockfalls", including an incident in 1915 that also led to a derailment, the RAIB said.
The stones which ended up on the track were washed out from the gravel-filled crest drain and surrounding ground, the RAIB report concluded.
The report said the “lack of an effective drainage inspection regime” meant that any such indications of future problems upslope of the catchpit would not have been detected.
Three months before the derailment, two members of Network Rail staff, based at its Perth maintenance depot, carried out a drainage inspection in the area using a handheld computer loaded with information from its drain maintenance database.
However, the inspection did not include the crest drain upslope of the catchpit – designed and constructed by the now-collapsed Carillion Construction Ltd – given it was not included in the database.
It said the design and construction of the drain, plus the "intended and actual" inspection processes, were among the main areas it considered as part of its investigation.
The RAIB also said that an earthworks examination report of the site in January 2017 concluded the drain was “flowing freely”.
The examiner who carried out it was not expected to open any catchpits, and and was not required to climb up the slope.
Last month, a taskforce led by Lord Robert Mair, emeritus professor of civil engineering at the University of Cambridge, to examine the management of railway cuttings and embankments, said that drainage across the rail network was “often inadequate”.
It said there should be dedicated drainage maintenance teams across all routes, with the updating of monitoring and surveillance methods, particularly on embankments and slopes deemed to be “potentially critical.”
It also warned that climate change meant periods of more intense rainfall and higher frequencies of extreme rainfall are likely.
The RAIB report said there was "near-continuous heavy rain" in the area between around 5:50am and 9am on the day of the crash, which caused "significant flooding”.
The 51.5mm of rain that fell in this period was almost 75 per cent of the monthly total in Aberdeenshire in an average August. But it was "dry and sunny" when the derailment happened at 9:37am on August 12.
The 6:38am service from Aberdeen to Glasgow was returning towards Aberdeen at the time of the accident due to the railway being blocked.
After striking the debris, the train derailed to the left, destroying a bridge barrier. Its power car and one of its four carriages fell down an embankment. All sections of the train suffered extensive damage.
The report also said the route manager did not convene an extreme weather action teleconference (EWAT) on receiving that day’s weather forecast, as he concluded it was not “sufficiently adverse.”
But the RAIB’s 29-page document stated that if an EWAT had taken place, it was unlikely the accident would have been avoided, given it did not require any “mitigation” at, or near, the derailment site.
It noted that at the time of the accident, Network Rail had no formal procedure that required an immediate review of operating restrictions after multiple weather-related events.
The company has since issued operational guidance for route controllers during periods of extreme rainfall, which involves risk assessments for individual areas of the railway.
Julie Clark-Spence, a partner at law firm Balfour+Manson, which is representing Mr Dinnie's family, said there was "a long way to go before any conclusion is reached”.
She said: "The interim report provides an indication that there are still many unanswered questions, which need addressed quickly before there is any form of closure for them.
"Losing a loved one in these circumstances is devastating, but not knowing the full extent of what happened merely prolongs the agony for the family."
Kevin Lindsay, Scotland organiser for train drivers' union Aslef, said: "Blame for the accident has been laid firmly at the door of Network Rail for failing to maintain the area around the track.
"It was the landslip – the debris washed onto the track – which caused the train to derail, with the subsequent loss of life, injuries and catastrophic consequences.
"We are urging Network Rail to examine every mile of track for which it is responsible, to ensure something like this can never happen again."
Transport secretary Grant Shapps said he looked forward to the full RAIB report and ensuring “lessons are learned from this tragedy”.
Andew Haines, chief executive of Network Rail, said: "We remain absolutely committed to learning lessons from the tragedy last summer that cost the lives of Brett McCullough, Christopher Stuchbury and Donald Dinnie.
"We welcome RAIB's interim report and we continue to cooperate with all ongoing investigations as we seek to understand what happened.
"We are being guided by world-renowned experts as we tackle one of our biggest challenges, adapting our transport system to cope with the long term changes in weather in the face of a rapidly changing climate."
A message from the editor: