A group of track workers “narrowly avoided” being hit by a train travelling at 125mph (200kmh).
The incident happened during an inspection of the West Coast main line south of Kirtlebridge in Dumfries and Galloway on 14 November.
There were no injuries but the Rail Accident Investigation Branch has undertaken a preliminary examination.
It looked at the circumstances of the incident and said a safety report would be published in due course.
The incident happened at about 09:00 and involved a Virgin train.
The RAIB has now looked into what happened and said it would issue a safety report looking at what lessons could be learned.
It said that should be available in the next few weeks.
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