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.
