Network Rail has angrily denied union claims that a signalling "blunder" during a strike could have resulted in a head-on collision between two passenger trains.
The Rail Maritime and Transport union said it had asked the Railways Inspectorate to investigate claims that a train was signalled into a platform at Edinburgh's Waverley Station on Tuesday afternoon at the same time as another train was being signalled out of it.
General secretary Bob Crow said: "If that is true, it amounts to a near-miss and it raises serious concerns about the competency of the managers Network Rail is using to do our members' jobs during the dispute."
Hundreds of signalling workers started a 24-hour strike at noon on Tuesday and the union said the alleged incident happened shortly after the walk-out began.
But Network Rail said its Scottish route director, David Simpson, had already carried out a thorough investigation into the claim and found there was no basis for the union's allegation.
A spokesman said: "There was no operational incident and the RMT is completely wrong. They are scaremongering for scaremongering's sake."
The row flared hours before the two sides were due to hold fresh talks in Glasgow to try to resolve the dispute over staffing rosters.
Hundreds of journeys were cancelled on Tuesday and commuters were hit by fresh travel chaos on Wednesday morning.
A second 24-hour strike is due to begin at noon on Thursday unless the deadlock is broken.
The union said it was bringing a "new formula" to the talks to try to avert any more industrial action.
