British Transport Police are appealing for witnesses to a disturbance on a train between Porth and Pontypridd.
The incident took place between 10.40am and 11am on Friday 20 April.
A man boarded the train and went straight into the toilet. When the train guard knocked the door to check on his welfare, the man became aggressive and began shouting and swearing.
Officers are keen to speak with anyone who was on board and may have seen what happened. Any piece of information, no matter how small, could prove vital to the investigation.
Were you travelling on this train? Did you see anything?
If so, please text 61016 or call 0800 40 50 40 quoting reference 192 of 20/04/2018.
Alternatively, contact Crimestoppers anonymously on 0800 555 111.