British Transport Police say that just before 2.50pm on Thursday 13 July, a woman arrived at Carmarthen station to wait for the 2.50pm train to Pembroke Dock.
Before her train arrived, a man sat next to her and engaged her in conversation, before sexually assaulting her.
When the train arrived, she boarded the service, the man followed her and sat next to her, blocking her in her seat.
He continued to talk to her before sexually assaulting her again.
The man left the train at Tenby at around 3.50pm.
Officers from British Transport Police say they would like to speak to the man in the image who they believe may have information that could help their 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 2300081083.
