PARIS — Europe's rail system is at the heart of its open borders, a way for passengers to travel on some of the world's fastest and most sophisticated mass transit with no need for passports or even ID.
The man who plotted to attack a high-speed train last week paid in cash, allowing him to avoid identifying himself, and boarded in Brussels, the European Union capital, with a small arsenal of weapons in his bag. The thwarted attack raises pressure to increase security in the 26 countries that allow travel without border checks.
The EU has ruled out tightening controls at the frontiers, leaving individual governments — and the train companies themselves — to resolve an international security challenge they say isn't limited to mass transit, even if the railways are an acknowledged target.
The attack Aug. 21 on the Thalys train from Amsterdam to Paris happened at the height of the summer travel season, when the cars were packed with tourists, including the three American friends — two in the military — who tackled and ultimately subdued the gunman. The attacker, identified as Ayoub El-Khazzani, selected the train, refusing a ticket for an earlier connection.
He had been flagged for links to radical Islamists by Spain, and German officials tracked him flying from Berlin to Istanbul in May.
Trains in Europe have been targeted for terrorist attack before, notably in London and Madrid. And Europe is not alone in its vulnerability. Rail networks in Japan and the United States operate similarly. The Eurostar connecting Britain to Brussels and Paris requires a passport and luggage checks, and China has X-ray checks on subways.
But the latest incident has left authorities puzzling over how to boost security without causing transport gridlock, imposing exorbitant costs on security services, or revising the painstakingly drawn-up agreement that allows passport-free travel.
''It is better to concentrate on concrete measures that we can put into place easily and that will actually improve security,'' said Nathalie Goulet, a French senator who led a commission investigating jihadi networks.
France hosts emergency talks in Paris on Saturday, with representatives from other high-speed international rail nations — Belgium, Britain, Germany, Italy, Luxembourg, the Netherlands, Spain, and Switzerland, as well as the European Union's top transport and interior affairs officials.
''The Thalys is a train that covers four countries. This has to be dealt with on a European level,'' Guillaume Pepy, the chief executive of France's SNCF rail network, told the BFM television network this week.
Without open borders, rail services could not compete with low-cost air travel. Indeed any security measure that might stop people easily hopping on an international train downtown could be the system's undoing.
''It is important not to act rashly or overreact,'' said Libor Lochman, executive director of the European railways umbrella organization, CER. ''The level of threat from terrorism is different in each EU country, so flexible measures are needed to adapt to changing security threats.''