A Bargoed woman was threatened and screamed at by a complete stranger as she made her way home on the train.
British Transport Police are appealing for witnesses to the incident, which happened on a train between Cardiff Queen Street and Gilfach, to come forward.
Officers are also looking to trace a woman who gave the distressed victim a lift home after the incident.
PC Yvonne Windsor said the incident took place on Wednesday, February 10 on the 10.35pm service from Cardiff Central to Rhymney.
“The victim, a 23-year-old woman from Bargoed, had boarded the service when it called at Cardiff Queen Street.
“Shortly after boarding, she was approached by a man who, completely unprovoked, began to verbally abuse and threaten her.”
Upon hearing the commotion, a nearby train guard intervened and the man was ordered to leave the train at Gilfach.
The man is described as white and in his forties. He is approximately 5ft 7in tall with a slight build, short, dark hair and dark features.
He was unshaven and wearing dark trousers, a black jumper with red, white and blue stripes and a black denim jacket. He was also in possession of a black suitcase on wheels.
It is believed the man boarded the train at Cardiff Central.
The victim later approached another female passenger and asked to borrow a mobile phone to call relatives.
The woman also offered to drive the victim home, which she accepted. Officers are keen to trace this woman as she could have key information that could assist the investigation into the incident.
PC Windsor added: “This was an extremely frightening and intimidating experience for the young woman, who was simply making her way home.
“We have made local enquiries into the incident and are trying to build a clear picture of what took place – to help us do this we need to hear from anyone who recognises the man described or who has any information that could assist our investigation.
“If you witnessed the incident then we want to hear from you. We are particularly keen to talk to the woman who helped the victim and drove her home as she could have key information that could help our enquiries.”
Anyone who witnessed the incident is asked contact British Transport Police on freefone 0800 40 50 40, quoting reference 657 of 10/02/10, or call Crimestoppers anonymously on 0800 555 111.