The theft happened at the end of June as the train travelled between an army logistics base in Miramas near Marseille and another base in Brienne-le-Chateau, 200 kilometres (125 miles) east of Paris, the source said.
Investigators found that the weapons were inside cases that were “neither sealed nor secured”.
Four rocket launchers and four cases of armour-piercing shells were taken, the French daily Le Parisien reported.
Authorities quickly identified the alleged perpetrator, a habitual freight train thief who was arrested at his home in Miramas, where the stolen weapons were also found.
Investigators are trying to determine whether the man just happened upon the artillery or if he specifically trying to get his hands on such weapons.
The defence ministry has launched an internal probe into “the conditions of the transport of munitions by freight rail” and will “propose measures… to reinforce their security,” a government source said.
This is not the first time a large quantity of army weapons has been stolen in France.
In July 2015, thieves broke into the Miramas base and stole at least 150 detonators and a stock of plastic explosives.
In September 2016, cases containing handgun components were stolen from the Istres military base in southeast France. Four people, including a master corporal, were charged with the theft.