PARIS — Days after a young Moroccan man was thwarted from an apparent plan to cause carnage on a Paris-bound express train, European officials confronted the deepening quandary of what additional steps they could take in the face of such attacks on soft targets, short of paralysing public spaces or even more intrusive surveillance.

Enhanced security and surveillance measures had filtered out the young man, Ayoub Khazzani, 26. However, by now, he was one of thousands of Europeans who had come on the radar of the authorities as potential threats after travelling to Syria.

The sheer number of militant suspects combined with a widening field of potential targets have presented European officials with what they concede is a nearly insurmountable surveillance task.

The scale of the challenge, security experts fear, may leave the continent entering a new climate of uncertainty, with added risk attached to seemingly mundane endeavours, such as taking a train.

In fact, the authorities in at least two countries already knew quite a lot about Khazzani before he surged into notoriety last Friday afternoon: He was on a French list as a security threat and Spanish officials told news media there that he had travelled to Syria — not in itself an offence, unless he had gone there for jihad.

Had he been living in France, a tough new surveillance law, approved at the end of last month by France’s constitutional council, would have probably turned up even more on him.

Yet, with all that the authorities already knew about him, Khazzani managed to board, unhindered, the heavily travelled Amsterdam-to-Paris high-speed train with a sack of weaponry, probably in Belgium, and was ready to inflict serious damage with dozens of rounds of ammunition, an AK-47, an automatic pistol and a box cutter.

If not for the fortuitous presence of three Americans and the help of a British and a French passenger in the train car, many could have died.

The three Americans described the attack at a news conference at the United States Embassy in Paris on Sunday. Yesterday, they were presented with the Legion d’Honneur —France’s highest honour — by President Fracois Hollande.

“We are now faced with unpredictable terrorism,” said a French security consultant and terrorism expert, Jean-Charles Brisard. “Terrorists henceforth will be choosing soft targets, those where there is little security,” said Mr Brisard. “And that’s why he chose a train — because there is little security.”

On Sunday, French antiterrorism officials continued to interrogate the suspect. A lawyer who spoke to him insisted to French news media that the man was “bewildered” by accusations of terrorism, saying that he had merely wanted to rob the passengers.

However, those explanations were dismissed by the authorities, as well as by one of the young American servicemen who tackled him, Alek Skarlatos, who said at a news conference that given the amount of ammunition the man was carrying, robbery could not have been his motive.

Systematic controls only on foreigners

The shortcomings of the French security list were highlighted on Sunday by the leader of the far-right National Front Party, Ms Marine Le Pen. She called for the expulsion from France of terrorism suspects on the security threat list, saying there were “serious weaknesses” with the system.

Some antiterrorism experts agreed that control of the movements of suspect individuals with residency permits across European borders — including those on lists — was weak. And that played into the inherent weaknesses in controlling rail passengers, they said.

Because of the European Union’s borderless frontiers, there are no “systematic controls on Europeans” or those holding resident cards, only on foreigners, Mr Brisard said. And that is the real problem, he added.

With determined jihadists, 40 million passengers daily and 100,000 trains, securing Europe’s rail networks is a challenge unlikely to be met anytime soon, if ever, according to security experts.

The problem is that train stations — 3,000 of them in France alone — are open spaces, largely uncontrolled, where non-passengers can mingle freely with those getting on board.

Baggage is checked in only a few places, and for a very few trains — for the cross-Channel Eurostar and for some trains in Spain after the terrorist attacks there that killed around 200 people in 2004.

Europe’s trains, and stations, and thousands of kilometres of train tracks are very different from the tightly controlled space in airports, the security experts said. And trains are on the way to becoming the logical soft target of choice, they said.

“Among the soft targets, the rail system will be a major one, because today, they are so unprotected,” said French terrorism and risk expert Bertrand Monnet. For terrorists, “for years, their symbolic target has been air transport, but that has become very difficult”.

Trains are an evident target, said Mr Monnet. “Millions of people would say, ‘It could have been me,’” he added. “The question is not whether, but when.”

The half-dozen big train stations in Paris are like villages, with a constant stream of unchecked humanity pouring through them every day. Decades old, they were designed with none of today’s security problems in mind. Sporadic patrols by armed soldiers constitute a check, but a very limited one, said the experts.

One million passengers a day take the French high-speed train alone, and three million take the suburban train network.

“Access to the platforms over the whole network, to train stations — these are open spaces. It’s not like airports,” said Mr Marc Ivaldi, a European transport expert at the Institute of Industrial Economics in Toulouse, France.

“Even if you are not travelling, you have access. It is a huge space and one that is very difficult to make secure,” Mr Ivaldi said. “You could put cameras in, but you can’t imagine a system like airports.”

To install airport-style metal-detector gates at train stations would totally block the flow of rail traffic, said Mr Brisard.

“You are dealing very much with a popular expectation that you can go to the station and go wherever it is you need to be quickly and without too much hassle,” said Mr Christopher Irwin, vice-chairman of the European Passengers’ Federation, a Belgium-based passenger-advocacy group.

Tighter rail security may not be feasible

Adding new security checkpoints and additional layers of passenger and baggage screening, experts said, would not only extend passengers’ travel time but also stretch the physical capacity of urban train stations.

“You can try screening everyone, but that is unlikely to be sustainable,” said Mr Irwin. “You probably couldn’t keep the transport system working if you did that. Stations simply haven’t got the space to accommodate the queuing that would be required.”

Until now, the only train service in Europe that systematically screens passengers and their luggage is Eurostar, which connects Britain to France and Belgium, as well as some high-speed lines in Spain.

Elsewhere, security systems operate on a more random — and often less visible — basis, relying on networks of surveillance cameras, uniformed or undercover police officers, and bomb-sniffing dogs.

The most plausible scenario is some return to the situation prevailing in France in the mid-1990s, after a series of Islamist attacks on trains and rail stations. For a brief period, military personnel patrolled inside the trains, and baggage was checked.

Mr Brisard noted that such patrols potentially have a much more dissuasive effect than the mere sight of armed soldiers at train stations.

THE NEW YORK TIMES