The girl was approached by a man on 16.38 service from Balloch to Airdrie on Saturday July 3.
British Transport Police said the man sexually assaulted her and made inappropriate remarks towards her.
He left the train at Charing Cross Station in Glasgow at 5.30pm with a group of men.
The suspect is white, in his 60s, around 5ft 7in tall and of large build.
He has grey, curly hair and was wearing a psychedelic print t-shirt with dark shorts and round sunglasses and has tattoos with writing and stars.
British Transport Police said: “At the time of the incident the service was very busy with passengers, officers want to speak to anyone who may have witnessed it to assist their inquiries.
“Witnesses or anyone with information can contact BTP by texting 61016 or calling 0800 40 50 40 quoting reference 506 of 03/07/21.