A woman believed to be in her 20s was threatened with a knife while on a Penarth to Bargoed train.
Detectives from British Transport Police are appealing for witnesses to come forward as well as the victim of the threat.
Officers were called at 4.14pm on Tuesday July 9 following a report of a passenger being threatened on the 3.17pm Penarth to Bargoed service.
It is believed the victim left the train shortly after the incident at Llanishen – and officers are appealing for her to come forward.
Detective Inspector Mark Cleland said: “This was a very serious incident and we are eager to speak to anyone who was on the train on Tuesday.”
“However, we are obviously particularly keen to trace the victim, who we believe to be a young woman in her early 20s. She was described as wearing a white dress with birds on it, and it is understood she got off the train at Llanishen.”
Officers are in the process of viewing CCTV from stations and on board the train, while investigators have also released a description of a passenger they would like to speak to in connection with the incident. He is described as white, aged around 15 to 16 and wearing a blue bandana, sleeveless maroon top and shorts.
DI Cleland said: “It is believed this person got on the train at Cardiff Queen Street around 3.35pm before alighting at Ystrad Mynach just after 4pm.
“I would urge anyone who thinks they may know him, or anyone who was at Cardiff Queen Street or Ystrad Mynach rail stations between 3.30pm and 4.10pm yesterday, to contact us as a matter of urgency.
“You may have seen or heard something – no matter how insignificant it might seem – which could help our enquiries. You can talk to us in complete confidence and your information could prove vital to our investigation.”
DI Cleland added: “I would reassure the public that incidents such as this are extremely rare. Many thousands of passenger journeys are made safely each year, and the railway in and around Cardiff remains a very safe place for passengers and staff.
“Officers from our neighbourhood policing team have been conducting patrols at stations and on board trains in the area, providing additional reassurance to the travelling public.
“We are continuing to make extensive enquiries and are asking anyone with any information to come forward.”
Anyone with information should contact British Transport Police on 0800 40 50 40 quoting background log B8/WCA of 10/07/2013.
Alternatively you can contact Crimestoppers anonymously on 0800 555 111.