PARIS — In what is emerging as the first serious test of the determination of President Emmanuel Macron to stick to his reform agenda, France on Wednesday endured the second day of a train strike that left stations empty, platforms deserted and millions of travelers frustrated.

French unions, blindsided by the pace of Mr. Macron’s labor and fiscal reforms, have been spoiling for a fight. The terrain they have chosen, the country’s heavily used rail network, has the potential to either make the president buckle by crippling France’s economy or infuriate its citizens and turn them against the unions.

Other French governments have foundered as train strikes have stretched on, and this has the potential to last until June.

The stakes are high. If Mr. Macron wins — he is trying to overhaul the train workers’ ultra-protected job status, among other targets — “he will be seen as a great French reformer,” said Guy Groux, a labor specialist at Sciences Po, the leading political science institute. Meanwhile, the prestige of a leading union, the hard-left CGT, will be at risk, said Mr. Groux.