Witnesses are being sought after two 15-year-old girls were deliberately filmed by a man on a train travelling between Gowerton and Swansea.
British Transport Police says the incident happened on the morning of Tuesday 4 August.
Officers believe similar incidents involving the same man have happened elsewhere on the railway network.
The man is also wanted in connection with an assault at Chesterfield railway station in Derbyshire on 10 April.
Police say a number of people are believed to have witnessed what happened on the Swansea train, and they are keen to speak to them.
In particular, they want to trace a woman who followed the suspect and then spoke to the two victims.
The woman who checked on the girls could hold the key
That woman has not yet come forward, and officers believe what she saw, and what the girls told her in the moments afterwards, could help the investigation.
Anyone who saw what happened, or has any relevant information, can text British Transport Police on 61016 or call 0800 40 50 40, quoting reference 256 of 4 August.
Information can also be given anonymously to Crimestoppers on 0800 555 111.
The Gowerton to Swansea line is one of the busiest short hops on the local network, carrying commuters and students into the city each morning.
British Transport Police is responsible for policing the railway network across England, Scotland and Wales, which is how a suspect can be linked to incidents as far apart as Swansea and Chesterfield.
No description of the man has been released.
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