A train derailment in Argyll which left eight people injured was "probably" caused by a landslide, British Transport Police (BTP) have said.
A spokesman said "big boulders" had been found on the railway line.
The Glasgow to Oban service derailed shortly before 2100 BST on Sunday, near the Falls of Cruachan power station by Loch Awe.
The train driver has told how he and other staff helped lead passengers to safety.
Both carriages caught fire and came to rest over a 15-metre embankment. All 60 passengers were safely rescued.
Driver Willie Dickson, a railwayman for 30 years, said: "It was a real team effort after the train came to a halt."
The 60-year-old said he had applied emergency brakes on seeing "two huge rocks on the track."
He added: "I stayed at the controls until the train came to rest. I then entered the front carriage and tried to calm passengers down before taking people by the hand and leading them off the train."
A spokesman for BTP said an investigation into the incident was under way.
"Network Rail engineers are just going on site to make the train secure," he said.
"At this stage there is nothing to suggest any criminality.
"There are a few big boulders lying about on the railway line. Early indications are it's probably been a landslide but investigations are under way."
Speaking to the BBC News Channel, passenger Paul Gibson described how the 1820 BST service from Glasgow to Oban had derailed.
"It felt like it was a few minutes or so, but it must have been only about 20 seconds," he said.
"Bags and shelves seemed to sort of start flying across the carriage.
"And then all of a sudden on either side of us on the outside big balls of flame managed to come across us."
Steve Montgomery, ScotRail's managing director, paid tribute to staff on board the train.
He said: "This incident could have been more serious but for their actions. They did a tremendous job in difficult circumstances are a credit to the company."
Mr Montgomery said ScotRail was assisting the RAIB and other agencies in understanding the cause of the landslip and its effects.
Strathclyde Fire and Rescue Service said it was alerted to the incident by a passenger on board the train.
A spokesman said the call sparked a "major deployment" of fire crews, with firefighters working to gain access to the carriages.
Strathclyde Police said the passengers on board were taken from the train to Cruachan Power Station. No-one was seriously hurt, although many of those injured were described as "walking wounded".
An air ambulance was also called in because of the remoteness of the area.
An ambulance spokesman said one person was taken to Southern General Hospital in Glasgow, three to Lorn and Islands District General Hospital near Oban, and four to Mid Argyll Hospital in Lochgilphead.
The derailment has had a major impact on the surrounding transport network.
In addition to the line closure, the A85 is closed at the junction with the A819, but there is still access to Loch Awe village. The A85 is also closed at the junction with the A828.
Engineers from Network Rail and First Scotrail are examining how to remove the two carriages from the embankment.
Insp David McEwan from BTP said that the front coach of the train was "sitting in a precarious position overhanging the roadway".
"The coach could slip further down the embankment on to the roadway," he said.
"This is obviously a major concern for the engineers at this moment in time."