British Transport Police (BTP) are appealing to witnesses for information about the offence, which happened between 10.45pm and 11.25pm on Wednesday 20 March on board the Edinburgh Park to Dunblane service.
A man is reported to have sat next to the victim, engaged her in small talk and then blocked her in her seat.
He then subjected her to a prolonged and repeated sexual assault.
The suspect and the victim left the train at Stirling and he continued to inappropriately touch her and follow her throughout the station.
He left the area a short time afterwards.
The man is described as white, slim to medium build, with short dark hair, and wearing a green jacket and chino trousers.
The statement added: “Officers would like to speak to anyone who witnessed the offence or who knows the identity of the man.
“They would particularly like to speak to a member of the public who was sat opposite the victim who may hold vital information.
“Witnesses can contact BTP by texting 61016 or calling 0800 40 50 40 quoting reference 436 of 21/03/19.
“Alternatively, call Crimestoppers anonymously on 0800 555 111.”
Sign up to our newsletter: enter your email in the box at the top of this article to get daily updates straight to your inbox.