The 21-year-old passenger fell asleep on a Paris commuter train after getting on at the Charles de Gaulle station in the west of Paris at around 8am on Sunday. She then woke up to find one man sexually assaulting her while the other was stealing her phone.
She managed to leave the train in a suburb north-west of Paris.
After sifting through surveillance footage and tracking the woman’s phone, police tracked the offender to a hotel in northern Paris, and arrested the 20-year-old on Wednesday.
The other attacker, aged 18, gave himself up to police once he learned they were looking for him.
Paris has seen several shocking incidents on public transport over the past years.
The city was forced to step up security in 2013 on its RER trains after a gang of robbers stormed a stopped train, and after a woman was raped in broad daylight on another train.
A poll published last year indicated that 47 percent of participants “often” or “sometimes” felt unsafe when using France's public transport system.
The study also revealed that women, young people, passengers on RER trains, and people living in the Île-de-France area around the capital feel the most unsafe when using public transport.
The survey showed that young people are more jittery than older people when travelling. Some 56 percent of the 18 to 24-year-olds said they felt uncomfortable on public transport, as opposed to 37 percent of the over 65-year-olds.
However, the overall percentage of people who are worried when on public transport has gone down by two percentage points compared to 2013 when a similar study was carried out.
Those polled said they felt much more at ease when police, security guards, or conductors were present.
Better lighting and video surveillance were among suggestions from those polled as to how travel could be made more secure.
Some 90 percent of those polled would appreciate more video surveillance and 37 percent would even be willing to pay more for their tickets to finance the equipment.
While 45.5 percent of the subjects stated to "have been afraid, at least from time to time, of being assaulted or robbed on public transport" only 3.4 percent were victims of assault or theft within the last three years.