A train struck a large piece of rail left on a track in Inverness following engineering work, the Rail Accident Investigation Branch (RAIB) has said.
The collision near Cradlehall on February 25 was the latest in a series of incidents where lines were left unsafe after engineering works.
The 53mph impact pushed the rail off the track, preventing the Virgin East Coast Inverness to London train from derailing.
Urgent safety advice has been issued by the RAIB.
It warned: “Over the last four years there have been a number of incidents in which railway lines have been returned to service in an unsafe condition following engineering work.
“In all such cases there is a real potential for serious harm to people on subsequent train services.
“In light of these incidents -and given the serious nature of the most recent incident – the RAIB advises Network Rail to take urgent steps to review the effectiveness of the steps it has already taken to address this risk and to implement any additional measures that are required.”
ScotRail Alliance, which Network Rail Scotland is part of, said it carried out a full investigation after the incident and had taken “immediate steps to highlight the seriousness to all our employees”.
The RAIB said there were three similar incidents in England between 2014 and 2017.