"When you're attacked, you defend yourself," Philippe Martinez, the head of the powerful CGT union said on Friday, the day after the publication of the government-commissioned report on the future of the SNCF rail network.
Railworkers will demonstrate to "defend public service, their jobs and their status", he told France Inter radio.
Other unions have also condemned the report but have yet to say whether they will join the 22 March protest.
Civil servants and local authority employees were already due to demonstrate on that day against President Emmanuel Macron's pledge to reduce their number by 120,000 within five years and expand the use of short-term contracts.
Open to competition
Macron has also promised to shake up the rail company and the report by former Air France and Areva boss Jean-Cyril Spinetta is certainly along the lines the president indicated.
Hailed as "complete and lucid" by Prime Minister Edouard Phlippe's office, the report says it is urgent to tackle the rail company's 48-billion-euro debt and suggests measures to prepare to open the network up to competition in 2019, as required by the European Union.
Among it 43 recommendations are:
An audit of branch lines, which receive 16 percent of funding to transport two percent of passengers, with the aim of concentrating on transport in and around urban centres and the TGV high-speed train;
Phasing out of railworkers' current employment status, granted to compensate for the demanding nature of the work in the past;
A two-year voluntary redundancy programme to end "overstaffing, which it manages with limited success";
Conversion of the SNCF Moblitiés and SNCF Réseau state enterprises into public limited companies, to comply with EU competition requirements;
The state taking over part of SNCF Réseau's debt, which grows by about three billion euros a year.
The head of Macron's Republic on the Move party, Christophe Castaner, welcomed the report, declaring that the government should "dare to tackle taboos and privileges", and naming an average retirement age of just over 57 years old as one of them.
Economy Minister Bruno Le Maire declared that reform was needed because the network is "heading for disaster".
A number of malfunctions linked to ageing technology have hit the network over the last few months.
Green campaigners defend rail
Trade unions unanimously condemned the report, although only the CGT has declared its intention of demonstrating against it so far.
"We're heading for the hunt for commercial profitablity, which goes against the principle of public service for all," Roger Dillensenger of the Unsa union told the AFP news agency.
Closing branch lines would "kill off rail freight", 40 percent of is transported by branch lines, according to Erik Meyer, the spokesman of another union, Sud-Rail.
They were not alone in their criticisms.
Communist Party leader Pierre Laurent said that Spinetta was aiming for a "general privatisation of the rail network" and Christian Estrosi, the right-wing mayor of Nice, spoke out against the closure of branch lines, while supporting changes to railworkers' employment conditions.
Réseau Action Climat, a coalition of environmental NGOs, called for the development of rail as an "alternative to the car and the airplane" to reduce carbon emissions, as required by the 2015 Paris climate change agreement.
And the Fondation pour la Nature et l'Homme, an NGO set up by current Environment Minister Nicolas Hulot, called the report "a threat to access to mobility for all, territorial cohesion and the climate".
The government is to hold a consultation exercise, which will include the unions, although the CGT's Martinez has complained that it will be an empty gesture.
With their capacity to disrupt commuter and long-distance traffic, the rail unions have spearheaded movements such as the protests against labour reform by the present government and its predecessor.
If Macron's government tries to put through many of Spinetta's proposals, it may spark a confrontation with France's most powerful and militant labour organisations.