The 17-year-old girl died during the night of Tuesday to Wednesday, St Gallen's public prosecutor said in a statement on Wednesday.
She suffered severe burns in the attack on August 13th when a man stabbed passengers and set fire to a Südostbahn (SOB) regional train on the railway line between Buchs and Sennwald in the canton of St Gallen.
Both the 27-year-old attacker and a 34-year old female victim died from their injuries a day later.
Four other people were injured, including a man on the platform at Salez station who pulled the attacker from the train when it stopped there.
Newspaper Blick 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.
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, as were two men aged 17 and 50, but they have been discharged from hospital.
Speaking to local paper the St Gallen Tagblatt, public prosecutor Roman Dobler said the motive for the crime had not yet been determined.
He would not comment on the previous claims in some Swiss media that the perpetrator hated women.
Police still haven't named the man, but Swiss media including Blick identified him as Simon S., citing neighbours describing him as tall, lanky and a "loner".