Ayoub El-Khazzani, 25, was carrying 270 bullets for his assault rifle and a bottle of petrol, prosecutor Francois Molins told reporters on Tuesday.
The Moroccan’s phone showed that he had watched a jihadist video shortly before launching the attack, Mr Molins added.
Prosecutors have now filed formal charges against him.
Mr Khazzani is accused of carrying out a “targeted and premeditated” jihadist attack.
He is also accused of firearms offences and “participation in a terrorist association with a view to organising one or several damaging crimes,” according to prosecutors’ documents quoted by the AFP news agency.
He was overpowered by passengers on the Thalys express train from Amsterdam to Paris on Friday. No-one died.
Three Americans and one Briton who overpowered the gunman were awarded medals for their bravery.
A US-born Frenchman, Mark Magoolian, and another unnamed Frenchman will also be given awards later for trying to tackle the gunman.
Police are calling for any information that could help locate 14-year-old Tavleen Kaur who was