The prime minister of France, Jean-Pierre Raffarin, has appealed for calm after
a bomb was found on a main railway line.
The device was found on track near Troyes
The state-run rail company SNCF said the device was found during an inspection of the line between Paris and the Swiss city of Basel.
A bomb disposal squad made the device safe, the government announced.
SNCF has increased security since a group calling itself AZF sent blackmail demands, threatening attacks.
The group sent notes to the French president and the Interior Ministry demanding $4m and 1m euros (£2.5m in total).
That prompted a search of France's entire rail network two weeks ago, which failed to find any traces of an explosive device.
SNCF on Wednesday ordered another search of the country's 32,000km (19,800 miles) network, but did not when it might take place.
Buried under track
The bomb was discovered shortly after noon (1100GMT), half-buried under a track near Troyes, about 150km (90 miles) southeast of Paris.
A container with nitrate fuel was found inside a plastic box about 20cm (8 inches) square, with six detonators wired to a battery.
The Interior Ministry said it was not
clear whether the device was capable of exploding.
The device did not resemble bombs described in AZF's previous threats, it added.
There have been no claims of responsibility.
It is the second bomb found on the rail network in two months.
An explosive device was discovered near Limoges on 21 February.
It was described as "very dangerous" by Interior Minister Nicolas Sarkozy.