Thousands of passengers were trapped overnight on high-speed TGV trains halted by electrical problems in southwestern France, some pleading for water, food or fresh air.
Frustrated travellers raised attention to their plight on social networks, posting images of children sleeping on floors and describing the challenge of staying masked for as much as 20 hours straight.
Several people were evacuated for medical reasons, broadcaster France-Info reported.
National rail authority SNCF on Monday apologized for "a series of electrical supply incidents" that started Sunday afternoon and halted traffic in southwestern France and disrupted travel from there to Paris.
Four high-speed trains, connecting Bordeaux with other cities in the region, were stuck on tracks into the night, with knock-on effects on other routes, SNCF said.
Passengers were taken back to their departure points overnight and Monday morning, then put on buses and other trains to eventually reach their destinations. The problems came as millions of French people prepared to return to work and school this week - many returned Sunday by train from August vacations.
Traffic isn't expected to return to normal until Tuesday morning, SNCF said. It announced an internal investigation into what happened.
SNCF will reimburse all passengers three times the cost of their tickets and distributed masks, water and food, the junior minister for transport, Jean-Baptiste Djebbari, said on BFM television.
He said 60 kilometers of track suffered damage to high-power overhead cables.
