The incident happened around 11:30 p.m. Wednesday at the Lombard-South Station on the Broad Street Line.
SEPTA police say 26-year-old David Dash approached the 20-year-old pregnant victim from behind and put a knife to her throat as she waited at a fare kiosk.
He then fled the scene after the victim was able to escape.
Dash was arrested on Sunday. He is facing charges of aggravated assault, simple assault and related offenses.
The woman was treated for a laceration to the neck.
Anyone with information about this incident is asked to contact the SEPTA Transit Police Department at (215) 580-8111.