Police are hunting a man who reportedly masturbated in front of passengers on a train from London Paddington to Cardiff.
The incident happened around 12.10pm on September 9.
"A person exposed themselves and was masturbating until they were challenged," a British Transport Police spokesperson said.
"They later left the train at Swindon."
Detectives on Wednesday released two CCTV images of a man they want to trace in connection with the incident.
"Officers believe the person in the image may have information which could help their investigation," a spokesperson said.
If you recognise them, or have any information, please contact BTP by texting 61016 or by calling 0800 40 50 40, quoting reference 2300106204.
Alternatively you can call Crimestoppers anonymously on 0800 555 111.
