victims: Angus and Margaret Mackay died at Halkirk crossing.
The decision not to hold a FAI was greeted with dismay by Donald Mackay, whose parents Angus and Margaret died when a train hit their car at the Halkirk crossing in Caithness on September 29, 2009.
Mr Mackay's uncle, also called Donald, was the third fatality in the accident, which prompted a UK-wide review by the Rail Accident Investigation Branch (RAIB) into the safety of open level crossings, which have warning lights but no barriers.
The Crown Office confirmed to The Herald an inquiry will not be held and that the case is "closed".
In a letter to Mr Mackay explaining the decision, David Barclay, senior procurator-fiscal depute in Wick, said Crown Counsel, which decides on prosecuting crimes in Scotland, felt Network Rail had taken "responsible actions" in addressing crossing safety.
"They note that the incident has received widespread publicity and that Network Rail, which owns and operates rail crossings, have taken responsible actions in response to issues identified within the RAIB reports," Mr Barclay wrote.
"They note that there have been improvements at the level crossing and that further improvements are to be implemented in the near future. They also note that Donald Mackay may bear some responsibility for the collision."
Donald Mackay, who is suing Network Rail, said: "The letter says Donald Mackay may bear some responsibility for the collision. I presume they mean my father Angus. That's one of the reasons they're giving."
An investigation into the tragedy by the RAIB concluded driver Angus Mackay had not seen the warning lights before driving on to the crossing.
Investigators found he had been advised to wear distance glasses on two occasions but had not done so. But their report also highlighted the condition of warning lights as an underlying factor in the accident, concluding that Network Rail "did not properly understand the risk at Halkirk level crossing" because it had not taken account of a previous accident there in 2002.
A Crown Office spokeswoman said: "The procurator-fiscal at Wick received a report in connection with the deaths of Donald Mackay, Margaret Mackay and Angus Mackay at Halkirk, on September 29, 2009.
"After full and careful consideration of all of the facts and circumstances, independent Crown Counsel concluded that it was not a case in which it would be appropriate to hold a FAI. The case is now closed."
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