There has been another incident involving a runaway train in an area in Cumbria where four men were killed six years ago, it has emerged.
The Rail Maritime and Transport Union (RMT) said a freight train rolled backwards for a mile from Shap Fell towards Tebay on Tuesday morning.
Four workers died when they were hit by a runaway trailer at Tebay in 2004.
Network Rail said it was happy to talk to the RMT about any "legitimate" concerns the union had.
The RMT is calling for better protection for workers but Network Rail said it would not compromise safety.
Union officials said only quick action by signalling staff in Carlisle prevented an accident.
A Network Rail spokesman said: "The Rail Accident Investigation Branch is looking into an incident in the early hours of Tuesday 17 August involving a freight train operated by DB Schenker.
"There were no people working on that section of railway and there were no passenger trains in the vicinity either."
The union called for all rail lookout patrols to be reinstated.
General secretary Bob Crow said: "This is just the latest runaway rail vehicle, following incidents on London Underground and in Scotland, yet Network Rail is playing fast and loose with safety in the name of cutting costs.
"We have been calling for secondary protection for track workers during track possessions since Tebay."
The Network Rail spokesman said: "Network Rail will not compromise the safety of its people."
He said lookouts were still used in some areas and workers were protected in other areas.
"Nonetheless, we are happy to talk to the RMT about any legitimate concerns it may have," he said.