PARIS (AFP) - The foiled attack on a crowded Paris-Amsterdam train provoked a range of reactions from passengers, but the military training of two young Americans who tackled the gunman proved decisive, psychiatrists say.

When a heavily armed man produced an AK-47 in the aisle of the packed train on Friday, it drew three kinds of responses: shock, attempts to save one's own skin and finally rare action that saw the attacker being overpowered and a bloodbath almost certainly being averted.

French psychologist and criminologist Jean-Pierre Bouchard said such situations had an "effect of total surprise".

"Those who witness and comprehend what's happening undergo enormous stress. They feel an imminent danger of death," he said.

"Some people are going to be shocked and won't be able to do much. Others are going to try to save their own skins by showing themselves as little as possible, and hiding. And some people are going to try to get away, but in this case it was an enclosed space," he said.

The suspect, 25-year-old Moroccan Ayoub El Khazzani, allegedly boarded the Thalys train in Brussels carrying two guns, ammunition and a blade in his luggage.

He first wounded a French-American traveller in his 50s, but he was prevented from almost certainly causing carnage by two off-duty US servicemen and their friend Anthony Sadler, who leaped on him, grabbed his AK-47 gun and restrained him even as he lunged at them with a box-cutter.

A British passenger then helped them tie up the gunman.

One of the American servicemen, National Guardsman Alek Skarlatos, has said the suspect did not appear to have had weapons training.

Mr Spencer Stone, who serves in the US Air Force, reached Khazzani first and was slashed in the neck and on the eyebrow and almost had his thumb sliced off with the box-cutter.

"Fortunately there were people who saw the same danger but instead of being stunned or hiding went on to take action," Dr Bouchard said.

"Here we had young American soldiers, who were trained and who had professional skills, and who reacted together," he said.

Anyone who has already seen or been trained to deal with such a situation automatically has a different reaction when such an incident occurs, Dr Bouchard said.

Another psychiatrist Nicole Garret-Gloannec said the American servicemen "had all the tools to allow them to react in an appropriate way".

Quite apart from their training, they simply "thought faster" about what was happening on the train, processing what was happening and deciding to take action.

"Given the weapons he had, they realised that they all were all goners."

The course of action they rapidly decided on "was the only way, and the best way, to save their lives", she said.

US President Barack Obama has hailed the "extraordinary bravery" of the three Americans, who will receive France's highest honour, the Legion d'Honneur, from President Francois Hollande on Monday.

Experts said the reaction of the 62-year-old British businessman Chris Norman, who will also be honoured by France for assisting the Americans, was fascinating because he had no military training.

"Chris Norman is interesting because he has admitted that at first he thought of hiding," Dr Garret-Gloannec said.

But when he realised that others had sprung into action, he came to their aid.

"He was an ordinary human being. He had no training, but he wanted to take part. He is courageous."

The first passenger on the train to confront the gunman was a 28-year-old Frenchman who worked in a bank in the Netherlands. He attempted to grab his gun as the suspect came out of a toilet cubicle.

He has so far chosen to remain anonymous.

Dr Michele Vitry, a clinical psychologist attached to the Paris Appeals Court, said: "It would be interesting to know if he does martial arts, or if he had personal attributes that allowed him to react without fear."

Not everyone's reaction was as heroic.

French actor Jean-Hugues Anglade, star of the films Betty Blue and Nikita and a passenger on the train, has strongly criticised two of the staff in the restaurant car for fleeing towards the locomotive without trying to help anyone.

But in the view of Dr Garret-Gloannec, they had "no training in how to react to this type of situation and they have no responsibility towards the passengers".

"Jean-Hugues Anglade also wanted to save himself and save those around him and that is completely understandable."

Mr Sadler appealed on Sunday to everyone to act as he and his friends had done if confronted with a similar situation.

"The gunman would have been successful if my friend Spencer had not gotten up. I want that lesson to be learned. In times of terror like that... please do something. Don't just stand by and watch," he told a press conference in Paris.