The government says it must move quickly to get the heavily indebted behemoth back on sound financial footing before passenger rail traffic across Europe is opened to competition starting next year.
The proposed changes, which would end guaranteed employment and generous pension benefits for new hires, have infuriated unions who say the government is trying to make them a scapegoat for the company's woes.
They are also incensed by Prime Minister Edouard Philippe's vow to implement the changes by decree, putting off any discussion in parliament until later this year.
Philippe Martinez, head of the CGT union, accused the government of wanting "to pick a fight" after meeting Philippe on Tuesday to present a rival plan for improving the SNCF's services and finances.
The CGT and three other unions have already called for a nationwide strike on March 22, and will meet Thursday to decide on additional rolling strikes aimed at derailing the measures.
The government says a debt load of nearly 47 billion euros ($58 billion) and a huge pension burden -- for decades drivers could retire in their early 50s -- makes operating French trains 30 percent more expensive than elsewhere in Europe.
It wants 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, and the government has denied union claims that the move is the first step toward privatisation.
But the SNCF will soon have competition on its tracks under EU directives that must be implemented in the next few years.
Transport Minister Elisabeth Borne told French daily Les Echos 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.