The incident happened on the 1.03am Cardiff Central to Swansea service at around 1.50am on Wednesday 1 March.
A man is reported to have physically assaulted two women on the Transport for Wales service. He left the train when it arrived at Swansea and headed towards the City Centre via the High Street.

(Image: British Transport Police)
British Transport Police say officers would like to speak to the man in the images, who may have information that could help the investigation.
Anyone who recognises him is asked to contact British Transport Police by texting 61016 or by calling 0800 40 50 40 quoting reference number 33 of 01/03/23.
Alternatively, call Crimestoppers anonymously on 0800 555 111.
(Lead image: British Transport Police)