The train involved in the Stonehaven disaster crashed into a landslip, causing it to derail and slide along the ground for 90 metres, investigators have found.
It then destroyed a barrier on the edge of a bridge leading the front power car and one carriage to fall down an embankment, the Rail Accident Investigation Branch (RAIB) said.
Two other carriages of the ScotRail train overturned.
The Aberdeen to Glasgow Queen Street service was stopped by a signaller after passing Carmont, near Stonehaven, because a landslip had been reported ahead of it, the RAIB said.
When it became apparent the train could not continue its journey south, the decision was taken to return it to Aberdeen.
It was routed back to Carmont where it switched on to the northbound line, and travelled for around 1.4 miles before hitting the landslip.
Driver Brett McCullough, 45, conductor Donald Dinnie, 58, and passenger Christopher Stuchbury, 62, all died in the incident.
The RAIB said it is collecting evidence to help it identify “factors relevant to the cause of the accident and its consequences”.
It added that the scope of the investigation is likely to include factors such as:
– The actions of the people involved
– Recent inspections of earthworks and drainage in the area
– Management of the risks of extreme weather
– Actions taken in response to previous safety recommendations
The RAIB did not give a timeframe for when it will publish its final report.