The strike was called by the CGT-Cheminots union over what its members consider to be low salaries and job insecurity.
Worst affected are travellers in southern France, where up to four out of five trains have been cancelled on some lines.
Popular lines have been affected too, with only one in two trains running between Marseille and Nice.
While it's a national strike, much of the rest of France probably won't notice the effects of the cancellations.
Rail network SNCF noted that the TGV and Transilien lines were running as normal, but added that 1 in ten TER and Intercité trains had been cancelled.
Those concerned about whether their train journey will be affected can check for more information here.
The strike will run until Friday morning at 8am.