A Moroccan man sentenced to life in prison for an attempted terror attack on a Paris-bound train in 2015 that was foiled by passengers, had his appeal rejected Thursday by a French court.

Their heroics were later immortalised in the Clint Eastwood film "The 15:17 to Paris".

Ayoub El Khazzani, now 33, was disarmed and pinned down by passengers, who included off-duty American soldiers, on the Thalys train from Amsterdam on August 21, 2015 and convicted five years later of "attempted terrorist murders".

The attack came as France was reeling from the jihadist murders at the Paris offices of Charlie Hebdo magazine in January 2015.

That massacre was the start of a wave of terror attacks on French soil, often by so-called "lone wolf" assailants claiming to act for the Islamic State or other jihadist groups, that left over 250 people dead.

Khazzani, who has been held in isolation since 2015, voiced "regrets" and "shame" before the verdict was read in the Paris courtroom.

But presiding judge David Hill said Khazzani had offered little new information to help investigators trying to understand how he prepared the attack.

"There's reason to doubt any effort at self-examination," he added.

Khazzani's sentence includes a minimum 22 years behind bars before any chance of parole.

He had admitted to boarding the train with an AK-47 automatic rifle and a pistol and 300 rounds of ammunition, saying he was looking to attack US soldiers.

He entered a toilet to arm himself and shortly after shot a passenger who tried to stop him. This alerted others to the attack including three holidaying Americans, two of whom -- Spencer Stone and Aleksander Skarlatos -- were US servicemen.

"We were just doing what we had to do to survive," Skarlatos said during the initial trial.

Khazzani had claimed to be acting under the orders of Belgian Abdelhamid Abaaoud, one of the masterminds behind the bloody November 2015 terror attacks on bars and restaurants in Paris that left 130 people dead.

Three suspected accomplices of Khazzani and Abaaoud, sentenced to prison terms of between seven and 27 years in the original trial, did not appeal their convictions.