PARIS (AFP) - Since the 9/11 attacks in New York, airports around the world have implemented tougher security checks for travellers, but the foiled attack on a packed high-speed train in Europe raises questions whether railway stations should also follow suit.

The suspected gunman, named as 25-year-old Moroccan national Ayob El Khazzani, boarded the Amsterdam-Paris express in Brussels on Friday with a Kalashnikov assault rifle, a Luger automatic pistol, nine cartridge clips and a box-cutter, investigators say.

Courageous intervention by a trio of young Americans who overwhelmed the attacker prevented a possible massacre.

French counter-terrorism officers were on Sunday questioning Khazzani, as it emerged he had been flagged by intelligence services in at least four European countries.

According to an initial probe, Khazzani has denied any intention of waging a militant attack, saying he had merely stumbled upon a weapons stash and decided to use it to rob passengers.

But he was flagged as a radical Islamist by intelligence services in several countries, and French investigators are focussing on an extremist attack.

Under French law, suspects in probes related to alleged terrorism can be questioned for up to 96 hours, which means Khazzani could remain in custody until Tuesday evening.

In the wake of the episode, Belgium said it would increase baggage checks and patrols on high-speed trains.

And France said its state-run rail firm, the SNCF, would introduce an emergency hotline to report "abnormal situations".

Experts say these are valuable tools in the fight against terrorism.

But at the same time, these fast-track measures also highlight an underlying problem: applying airport-style security to railway stations is almost impossible.

"The idea of extending the airport system to railway stations today isn't something that I can call realistic," SNCF head Guillaume Pepy, said.

"There's a choice - you either have comprehensive security or low (transport) efficiency."

Railway hubs were built in the 19th and 20th centuries when today's type of terror threats were inconceivable.

As a result, main stations are designed to have a maximum free flow of people on and off trains, with sometimes dozens of departures or arrivals at peak times.

They are served by a network of smaller stations - 3,000 of them in France alone.

"Airplanes leave from a specific place - you can build a security apparatus around it," said Mr Raffaello Pantucci, of the Royal United Services Institute in London.

"It's just not possible to do that with trains. You would have to do that at every station."

Retrofitting the railway network - nationally and internationally - so that it meets airport-style criteria would be astronomically costly, said Mr Marc Ivaldi at the IDEI research institute in Toulouse, southwestern France.

"The task is strictly impossible in the immediate future," he said.

In the absence of regular station screening, watchdogs have to rely on high-visibility patrols, spot checks, enrolling the public in campaigns for greater vigilance, installing video surveillance cameras and beefing up coordination between police and railway security.

The notable exception in Europe for security screening is the cross-channel Eurostar service that connects Britain to France and Belgium.

It requires passengers to arrive at a purpose-build terminal around a half-hour before their train departs and subjects them to pre-departure baggage and ID checks.

But this is largely due to Britain not being part of the joint Schengen passport-free zone, as well as security measures required for travel through the Channel Tunnel.

The baggage screening, too, is less demanding than at airports. Items are X-rayed but liquids are not confiscated.

"The key need is to secure the Thalys and a certain number of high-speed trains," said Mr Ivaldi, referring to the service on which Friday's attack occurred.

In Italy, passengers departing from some major stations have since May 1 been subject to security checks before boarding.

Railway security has been tightened too, in Spain.

Coordinated bomb attacks on on Madrid's commuter rail network on March 11, 2004 left 191 dead and 1,800 injured, in the deadliest terror attack in Spain's recent history.

Today, passengers boarding long-distance trains have their luggage checked.

A Spanish counter-terrorism source said Khazzani had lived in Spain for seven years until 2014.

During his time in Spain, he came to the attention of the authorities for making hardline speeches, and was once detained for drug trafficking, according to the source.

Spanish intelligence services say he went to France, from where he travelled to Syria, but the suspect has reportedly denied going to the conflict-ridden country where the Islamic State in Iraq and Syria controls swathes of territory.

A source close to the French probe, meanwhile, said he "lived in Belgium, got on the train in Belgium with weapons likely acquired in Belgium. And he had identity papers issued in Spain".

French Interior Minister Bernard Cazeneuve said Spanish intelligence services had tipped off France over his ties to "radical Islamist movements", but it is unclear whether he lived in France at any time after leaving Spain.

German security services, meanwhile, flagged Khazzani when he boarded a flight from Berlin to Istanbul in May this year and in Belgium, Justice Minister Koen Geens confirmed Khazzani was "known" to the country's intelligence services.

France has been on high alert since Islamist gunmen went on the rampage in January, killing 17 people in Paris, and the authorities have since thwarted several other attacks.