By Caroline Wyatt
BBC correspondent in Paris
Step by step, and inch by inch, French railway workers walked 32,500km of track, searching for any sign of a bomb on or next to the rails.
All of France's 32,500km of track is being checked
They uncovered nothing, but the French authorities were taking no chances.
An unknown group calling itself the AZF claims it has laid 10 bombs on the rail network.
It warns they will be set off if the government doesn't pay a multi-million pound ransom.
"We know nothing about this group, but we take their threat very seriously," said Interior Minister Nicolas Sarkozy.
"Based on information from them, we discovered an explosive device under the railway near Limoges on the 21 February," he said.
"It was checked, and proved to be very dangerous."
Police had kept in contact with the group via the personal ads in a French newspaper, with the blackmailers using the alias Big Wolf.
But on Monday, an attempt to hand over the ransom failed when police were unable to find the drop-off spot.
The French railways are doing their best to avoid creating panic among passengers - searching the tracks and putting every station across France on high security alert.
But at the same time, they remain determined to keep services running as normally as possible, while police try to hunt down this mysterious group.
French trains seems to be running as punctually as ever, despite one false alarm at a station yesterday.
Here in Paris, passengers say they're determined not to give in to terrorism.
But there's one question on everyone's mind - who are the AZF?
Are they simply criminals who want cash, or is there a more sinister agenda behind these threats?