A train travelling at 125mph almost struck three track workers after a breakdown in communications, rail accident investigators have concluded.
The workers jumped clear less than one second before the train passed them on the West Coast main line in November.
The Rail Accident Investigation Branch report said one of the lookouts had not known he was meant to raise the alarm.
It said the lookout team had used "informal language" rather than following the proper protocols.
A radio-based lookout-operated warning system was being used because of the high train speeds and the curved track south of Kirtlebridge in Dumfries and Galloway.
The system uses flashing lights and a siren to warn staff that a train is approaching.
However, there had been no warning before the train came round a bend at the spot where the inspection was taking place.
The driver sounded the horn to warn the workers and used the emergency brake, enabling the inspectors to jump clear less than a second before the train passed. No-one was injured in the incident.
The report said there had been a breakdown in communication between the system's controller and one of the two lookouts.
"There is conflicting evidence about the words spoken during the conversation... during which lookout (north) and the controller reached a different understanding about whether the lookout duties had already commenced," it said.
The controller said he had told the lookout that he was "up and running", while the lookout said he thought the controller was going to contact the other lookout and then phone him back.
The report said the team had been "following their normal practice of using informal language, rather than the formal communication protocol mandated by Network Rail.
"It is certain that their conversation did not result in a clear understanding between the staff involved."
The RAIB said it was likely that if they had used the formal words which were advised in their training, the lookout would have known he was expected to send a warning.
The report also said that guidelines for the use of the lookout system were last updated in 2009, despite the introduction of upgraded equipment in 2010 and 2018.
It said that while this was not a factor in the Kirtlebridge incident, one of the existing protocols has "the potential to cause unnecessary risk to track worker safety".
"Network Rail's national workforce safety team has stated their intention to address this issue and are considering the most efficient way to ensure that staff use the simpler protocol," it said.