"I wouldn't call it a fusillade, because even in the next wagon we didn't hear any shots," said Margaux, who declined to give her last name. She said passengers remained "relatively calm," and some showed no concern until police boarded the train.

Europe's major rail stations, such as Paris' Gare du Nord and Brussels' Gare du Midi, are patrolled by soldiers armed with rifles, but passengers can board most high-speed trains without passing through metal detectors or having their bags searched.

One exception is the Eurostar between Paris and London. Passengers on those trains must pass through a metal detector and have their bags scanned as well.

Thalys is owned by the French and Belgian railways and operates high-speed trains serving Paris, Brussels, Amsterdam and Cologne, Germany.

French authorities have been on heightened alert since Islamic extremist attacks in January left 20 people dead, including the three attackers.