The incident happened on the 23:09 service from Dunblane to Edinburgh on 16 December shortly after the train left Larbert railway station.
A man who was sitting in the seat next to the girl placed his hand on her legs twice.
She was able to alert a conductor who helped her to move seats.
The man was described as white, about 5ft 11in tall, in his late 20s, with strawberry blonde hair.
He was wearing a Christmas jumper and jeans at the time.
PC Andrew McAinsh, said: "Everyone should be able to travel on the railway network without experiencing unwanted sexual behaviour.
"I would like to commend the victim for reporting what happened to her.
"She was very shaken by the incident and an investigation is under way to identify the offender."
A 17-year-old girl has been left "very shaken" after being sexually assaulted on a train by a man wearing a Christmas jumper.