At issue is a proposal to divide the state rail network. SNCF, into three different establishments that the unions say would mean a breakup of the company, Radio France Internationale reported Wednesday.
The government opened debate on the reform plan at the end of last month.
Eurostar and international services to Belgium, the Netherlands and Germany will run normally Thursday but service to Switzerland will be cut in half and only one in three trains to Italy will run.
Paris regional trains will also be affected, with only one in three running on Line B to Charles De Gaulle and Orly airports.
The strike is scheduled to end Friday morning.