An estimated 20 robbers carried out what appears to have been a well-coordinated strike at 5am on Sunday, in the southern suburbs of Paris.
A police source told the Liberation newspaper that the group divided into two and then robbed passengers on both the upper and lower deck of the train.
The robbery began as the train left the platform at Juvisy-sur-Orge station in Essonne, heading towards Paris.
The thieves fled the train with their loot at the next station.
Only one woman has reported the matter to police, claiming that her mobile phone was stolen, but BFM TV reported that up to seven people were robbed.
Police are now calling for other victims and witnesses to come forward.
The local mayor and a police union claimed that France's raised anti-terror alert which has seen officers re-stationed at sensitive areas meant there were areas that have been left under-resourced.
'Police are understaffed due to terror alert'
"We cannot be everywhere. On that morning there was not one police officer on duty at Juvisy station. If the passengers had called police during the robbery I am not even sure we could have sent someone," said Claude Carillo from the police union Alliance.
"We were already understaffed before the raised terror alert (Plan Vigipirate) and now the area around Paris (Grand Couronne) is totally deserted.
Nevertheless the RER service, which links Paris to the surrounding suburbs and Ile-de-France region, has a reputation for being a crime hotspot.
In 2013, security was stepped up on the line after 20 robbers struck at Grigny station. Armed with pepper spray, the gang threatened passengers before taking their personal valuables in what witnesses described was a "highly organized" attack.
In January this year, a young woman was sexually assaulted by two men after she fell asleep while riding the RER near Paris.
And a recent study reports that half of French people feel unsafe when using public transport.
After the recent attack, opposition politician Valérie Pécresse once again called for more CCTV to be installed at RER stations.