The victim was on a service to Denmark Hill on December 3 at around 8.30pm when the man boarded the train.
As he threatened to punch her and then sexually assaulted her, she tried to escape despite the man allegedly blocking her exit.
Another passenger on the train stepped in and the man then relented and allowed her to pass.
Officers believe the man in the CCTV image may have information that could help with their investigation.
Anyone who recognises them is asked to contact BTP by texting 61016, or by calling 0800 40 50 40 quoting reference 778 of 3 December.
Information can also be given anonymously to Crimestoppers on 0800 555 111.
