The TGV train heading from the northeastern city of Colmar to Paris was delayed as a result of being stopped for security forces to intervene, a spokesman for national rail operator SNCF said.
“The situation is finished and under control. The individual has been arrested,” he told AFP.
The local police station said the 24-year-old “was threatening to blow himself up” and asked the passengers if they believed in God, but had neither weapon nor explosives on him.
The train carrying 280 people was held up shortly before 8am in the northeastern town of Louvigny to allow police to intervene and detain him for “threatening to commit an attack”, it said.
Some 100 members of the security forces and specialised sniffer dogs carried out a “thorough search” of the train but found nothing, before the train was allowed back on its way at around 10.20am.
Investigators said the man, who had already made a similar threat on a train in the northeastern city of Reims last year, was undergoing a “psychiatric evaluation”.
A police source said that the man was initially detained by a policeman who was travelling on the train as an ordinary passenger under a system that allows them to travel for free if they signal their presence to the conductor.
This set-up “each day proves its efficiency for the safety of French people on transport”, Interior Minister Gerald Darmanin said.