An accident in which a train was derailed by a landslip on the West Highland Line is being examined by the Rail Accident Investigation Branch.
Hundreds of tonnes of mud and stone slid down from a hillside on to the track between Lochailort and Glenfinnan just before 07:00 on 22 January.
The five passengers and two crew on board escaped injury.
The probe will examine how the line is protected from landslips and procedures during periods of rain and snow melt.
Last month's incident happened during rain and also thaw following a period of heavy snow and low temperatures.
ScotRail services were suspended between Fort William and Mallaig for about a week while engineers worked to free the train and clear the line of debris.
The Rail Accident Investigation Branch (RAIB) said the accident happened in darkness following a period when "significant" snow melt occurred at the same time as "moderately heavy rainfall".
A fence which had been recently installed to protect the railway track from individual loose boulders rolling down from the nearby slope was destroyed in the landslip.
The RAIB said its investigation would seek to identify the sequence of events which led to the accident.
It will also consider the arrangements for protecting the railway from landslip debris at the site and Network Rail's procedures for managing the combined effect of rain and melting snow.