France took virtually every precaution to make sure that its capital would be secure when all eyes were on the city for the opening ceremony of the Paris Olympics.

But the saboteurs who disrupted travel for hundreds of thousands on Friday did so by striking far outside the capital, targeting a rail network so vast, experts say, that it is impossible to secure every foot of it. And they knew exactly where to strike to cause maximum chaos.

“The S.N.C.F. faces attacks on its network every year, every month even,” said Julien Joly, a transportation expert at the Wavestone consulting firm, using the abbreviation for France’s national railway company.

He added: “But never in these proportions, and never in such a coordinated way. Unfortunately the network is so vast, you can’t ensure a 24-hour security presence everywhere.”

France has a very dense train network, with 28,000 kilometers of tracks (about 17,400 miles) used by 15,000 trains every day. Much of it is separate from the high-speed train network.

But the TGVs, as the high-speed trains are known, whisk thousands of passengers across the country daily, and are one of France’s most prized national infrastructures — a symbol of its technical know-how.