Two further 48-hour strikes are due to cause commuter chaos across Scotland, it has been announced.
Rail staff are angry over a change in their shift patterns
Strike action is to continue over working hours and the RMT will ballot UK members about joining the dispute.
Staff in Scotland will strike from Friday 16 March to Sunday 18 March and on Monday 19 March to Wednesday 21 March.
Network Rail called for further talks and said they would try to minimise disruption to passengers.
The RMT executive announced plans to ballot more than 4,000 signalling and supervisory members throughout Britain.
The row is over proposed changes to shift patterns and centres on how a 35-hour week, agreed last July, is to be implemented nationally. It has been largely accepted in England and Wales.
First strike action between noon on Friday 16 March and Sunday 18 March
Second strike action between noon on Monday 19 March and Wednesday 21 March
The dispute is about working hours. RMT claimed it has an agreement for its members to work a three-day, 35-hour week but they said this has not been implemented by the employers.
Network Rail said local agreements had been reached across 80% of Britain, including Scotland.
Strike action between Wednesday 7 March, and Friday 9 March, caused widespread disruption across Scotland.
RMT general secretary Bob Crow said: "Our members are adamant that the deal we signed last July should be implemented properly and in full, and after consulting reps around Scotland we will today be informing Network Rail of two further 48-hour strikes.
"Network Rail have been drafting in scab managers from around the country in their attempt to undermine our members' strike, and evidence is emerging of serious safety breaches that affect our members throughout Britain.
"The 35-hour week deal was a UK-wide deal, and the company's blatant reneging on the assurances they gave us over its implementation also has serious implications for our members throughout Britain.
"As a result the RMT executive will also now be informing Network Rail of our intention to ballot signalling and supervisory staff for strike action across the UK rail network."
David Simpson, Network Rail route director for Scotland, said union claims about safety were "completely without foundation".
He added: "We categorically deny that any safety risk has been caused to passengers during this strike.
"Once again, the RMT is using scaremongering tactics to justify their unnecessary, unjustified strike action.
"Our message is crystal clear, the only way to resolve this dispute is through discussion, and our door is wide open.
"We once again call upon the RMT to join us at Acas so we can settle this dispute. We are setting up a meeting on Monday with Acas and extend an invitation to the RMT to join us."
First Minister Jack McConnell said rail passengers would be "understandably furious" at the new strike dates.
He said: "There will be a suspicion everywhere in Scotland that the trade union leadership seems set on strike action regardless of any effort by management to discuss a solution.
"Effectively, their actions amount to six days of unnecessary disruption for both the travelling public and Scottish businesses.
"I reiterate my call to both parties to get round the table as quickly as possible with the intention of resolving this dispute.
"The trade union leadership are totally irresponsible in organising further disruption before there are any more talks."