A suspicious object on a railway line north of Paris hit rail services, including the Eurostar, but it turned out to be an oxygen canister.
French rail lines were searched after an earlier blackmail demand
The bomb disposal experts who examined the object marked "AZF, do not touch" dismissed the threat as a bad joke.
Earlier this year, a group calling itself AZF threatened to detonate devices if it was not given a ransom.
High-speed TGV services as well as the Eurostar were delayed while the discovery was investigated.
Traffic has now resumed on the section north of Paris after it became clear the object did not present any danger, a spokesman for the national rail company SNCF told BBC News Online.
Eurostar cancelled one service and others were delayed.
French railways have been on a higher state of alert since the AZF blackmail demand warning of bombs on the line.
The group sent notes to the French presidency and the Interior Ministry demanding $4m and 1m euros (£2.5m in total).
Earlier on Thursday a man was held in the UK under anti-terror laws, but this did not affect Eurostar, the firm said.
The incident was believed to involve a rail line between Dover and Folkestone which is not used by the Eurostar train.