At around 5pm on 21 January, a teenage girl was standing outside Basildon station when a man approached and made inappropriate comments to her.
She then boarded a London-bound train, and during the journey, the same man sat next to her and sexually assaulted her.
Officers are keen to speak to the man in the images as he may have information that could help with their investigation.
Witnesses, or anyone with information, can contact BTP by texting 61016 or calling 0800 40 50 40 quoting reference 434 of 21 January.
Alternatively, call Crimestoppers anonymously on 0800 555 111.
