The rolling strikes will be carried out on two days out of every five until 28 June unless the government drops its plan, which includes stripping new recruits of jobs-for-life and other benefits, the CGT said after a meeting of the company's four main unions.
"The unions see no will to negotiate on the part of the government... and take responsibility for an intense and long-lasting conflict," Laurent Brun, head of the CGT Cheminots rail union, said.
The government plans to push through its reform plans by decree in the coming months to improve services that cost 30 percent more to operate than elsewhere in Europe.
It says it must move quickly to get the SNCF back on sound financial footing before passenger rail traffic across Europe is opened to competition next year.
"This is a necessary, indispensable reform," Transport Minister Elisabeth Borne told BFM television on Thursday.
"My hope is not a test of strength, my hope is for negotiations."
But unions say depriving new hires of a special status which offers job guarantees and extra pension rights is a red line.
They have already called for a one-day nationwide strike on March 22 leading to speculation in the media about whether France will face disruption comparable to the paralysing wave of walkouts in 1995.
"It's bad news, a bad thing for the 4.5 million French people who take the train every day," SNCF boss Guillaume Pepy told the TF1 television channel.
A poll by the Odoxa-Dentsu Consulting group published on March 1 showed that 58 per cent of French people thought a strike would be "unjustified" and 72 percent agreed with taking away railway workers' special status.
But 64 per cent of respondents feared that the country was going to be "totally blocked" as in December 1995.
The SNCF has a debt load of nearly 47 billion euros and a huge pension burden -- for decades drivers could retire in their early 50s.
The government hopes to introduce more flexibility in working conditions and contracts while pledging to invest 3.6 billion euros in infrastructure over the next 10 years.
The overhaul would also turn the SNCF into a publicly listed company, though the state would own 100 percent of the shares.
Foreign competition will soon be on French tracks, such as Germany's Deutsche Bahn, under EU directives that must be implemented in the next few years.
Borne told Les Echos newspaper on Wednesday that rival operators could start running on France's vaunted high-speed TGV lines under an "open access" system starting in December 2020.
- with AFP