Quebec coroner Catherine Rudel-Tessier said 20-year-old Audrey-Anne Dumont was not texting on her phone, but was simply daydreaming when she fell.
It was morning rush hour last April 19 at Monk metro station and as the train approached, Dumont was the last person left on the platform.
Rudel-Tessier said, in the surveillance video of what happened next, Dumont accidentally stepped between two cars and was dragged to her death as the train started to accelerate.
"She wasn't texting or talking on the phone. What we suspect is that she was simply listening to music and was distracted," said Rudel-Tessier.
- Friends hold vigil for young woman killed in metro fall
Rudel-Tessier said new metro trains, due in service next year, will prevent such accidents because there will be no spaces between cars.
However, she said the metro would be even safer if it had a transparent plastic wall between the platform and the trains with sliding doors that match up with train doors and open only once the train is stopped.
"It would prevent suicides, accidental falls and objects falling on the tracks," said Rudel-Tessier.
The STM said it plans to install such walls, but it will take several years.