Rain and a rapid thaw of snow caused a landslip that derailed a train and led to the closure of part of the West Highland Line for several days.
The Rail Accident Investigation Branch (RAIB) found that Network Rail’s processes for managing landslip risk "did not take account of the hazard caused by rapidly melting snow".
Investigators said it was "unlikely" that a greater understanding of snowmelt risk would have avoided the accident "but it could avoid or mitigate an accident in other circumstances".
Five passengers and two crew were on board when the train came off the track at Loch Eilt as hundreds of tonnes of mud and stone slid down from a hillside on January 22. No-one was injured.
The damage closed the route for a week while the Glasgow-bound train was recovered and track repaired.
The Rail Accident Investigation Branch (RAIB) said a protective fence installed to trap falling rocks was "overwhelmed" by the landslip between Lochailort and Glenfinnan.
Some diesel fuel escaped from the damaged train and was carried by flowing water into a drainage channel, the RAIB said.
Investigators said that due to the inaccessibility of the site, pollution control measures were not put in place until later the following day, and by this time some diesel fuel had entered nearby Loch Eilt.
The RAIB recommended that Network Rail promote the development of weather forecasting processes to "take account of the risk due to snowmelt and ground thaw".
A Network Rail spokesman said: “The safety of passengers and staff on our railway is of the highest importance.
"We will closely review the RAIB findings and have also conducted our own internal investigation into this incident.
"We spend around £20m a year on earthworks and drainage projects in Scotland to try to prevent landslips and flooding and carryout regular inspections in locations where landslips are a known risk.
"We're also exploring how new technologies and monitoring systems and improved weather forecasting techniques can be used to improve how we manage weather events.”
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