The state railway company SNCF announced shortly afterwards that police and rail workers would again comb the country's railway network.
France's railway network - 32,000km of track along parts of which high-speed trains race at more than 200km an hour - had last been checked in a fruitless search for bombs on 4 March.
The bomb found on Wednesday with several detonators "strongly resembled" a previous bomb planted by a shadowy group calling itself AZF which has threatened to blow up parts of France's rail network unless it is paid millions of euros.
The interior ministry issued a statement saying a SNCF employee found the bomb 200km south-east of Paris in the town of Montieramey. Rail traffic was suspended in the area, which was sealed off by security personnel.
Anti-terrorist magistrates immediately launched an investigation but are yet to establish a link between the bomb and the AZF threat.
The bomb discovery came on the same day that French President Jacques Chirac was in Madrid to attend a memorial mass for the more than 200 people killed in train bombings in Madrid on 11 March.
French authorities have been on high alert for any plots targeting trains since AZF first surfaced in February with its threat and blackmail demand.
To show the seriousness of its claim, AZF tipped off French police last month to the location of a previous bomb, considered by police to be "sophisticated, worthy of an explosive expert".
After an exchange of messages via letters and newspaper classified pages, and an unsuccessful attempt to drop off the money, AZF apparently disappeared – but not before increasing its extortion demand and warning it would strike railway targets if it was not paid.