The 10 unions have already staged strikes on four days in February and March and had already announced they would stop work on 7, 10 and 11 April.
On Wednesday, after just 45 minutes of talks with management, they said they would stage a further strike on 17-18 April and 23-24 April in pursuit of their demand for a six-percent pay rise.
French rail workers will also be on strike on the 18, 23 and 24 April.
Management had made "no realistic proposals", the unions said in a statement, claiming that their industrial action has already cost the company 100 million euros.
The company offered pay rises for employees whose incomes had been lower than the inflation rate between 2011 and 2017.
It says the first day caused the loss of 26 million euros and claimed that accepting the union's demand would cancel out the savings made by a cost-cutting programme that began in 2011.