"In the course of the night, another victim -- a 17-year-old woman -- succumbed to her injuries in hospital," the prosecution office in the eastern canton of St Gallen said in a statement.
The woman was one of six people injured in the August 13 attack on a moving train near Salez station, not far from the eastern border with Liechtenstein and Austria.
A 27-year-old Swiss man used a flammable liquid to start a fire in a carriage and began stabbing passengers.
Police have not revealed his motive, but have said it did not appear to have been politically motivated or an act of terrorism.
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A 34-year-old woman, reportedly a Croatian national and mother of two, died the next day of stab wounds.
The Blick daily has identified the 17-year-old victim as Melissa B., a dentistry student who lived in Liechtenstein, and reported that she had been burned over 75 percent of her body in the attack.
"She was burning from head to toe, like a torch," he said.
A six-year-old girl reportedly remains in hospital after the attack, but is not in a critical condition.
Her 43-year-old mother was also injured in the attack, as were two men aged 17 and 50, but they have been discharged from hospital.
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