Dan Stessel, a NJ Transit spokesman said the woman was leaving the train at 2:09 a.m. when she was attacked by a man who forced her to perform a sexual act.
Stessel said the assault happened at the bottom of a staircase that leads to another platform. He said the attacker did not have a weapon but threatened to use force.
The suspect fled on foot and the victim called 911 from inside the Bound Brook station. She was taken to a local hospital.
Investigators are looking at surveillance video from the Bound Brook station and other stations on the Raritan Valley Line.
The suspect is described as a 5'9" to 6' tall Hispanic male with a thin build.