Eyewitnesses told officers that the 33-year-old man appeared to be in a drug-induced haze when the row erupted on the train travelling from the capital New Delhi to the city of Amritsar.
The man was attacked by male family members of the pregnant woman, according to accounts given to police.
"A French citizen who was found bleeding and unconscious on the platform at Karnal train station on Sunday died today in hospital," Ved Pal, assistant sub-inspector of Karnal police in Haryana state, told AFP.
"Eyewitnesses said the man began harassing a pregnant woman in the carriage. Her male family members got into a violent scuffle with him," Pal said.
The man was taken to a hospital in the city of Chandigarh, 145 miles (240 kilometres) north of Delhi, but never regained consciousness and died on Thursday.
Police said an investigation had been launched and that the train's ticket collector had been arrested for failing to inform the authorities about the incident.
"We have taken a blood sample from the victim to verify whether he was indeed on drugs," Pal added.
The French embassy in Delhi declined to comment.