A 75-year-old pensioner was punched twice in the face after asking a fellow passenger to stop smoking on board an Elizabeth Line service.
Detectives at the British Transport Police said the victim challenged a man who was smoking on an Elizabeth Line service from London to Twyford shortly before 11.45pm on July 8.
Smoking and vaping is banned on TfL services and in TfL stations.
The man then threatened the victim and then punched him twice in the face as he left the service at Maidenhead.
The victim sustained a black eye and a cut to his eyebrow in the attack.
Police believe the man in the CCTV image may have information that could help their investigation.
Anyone who recognises him, or has any other information, is asked to contact British Transport Police by texting 61016 or by calling 0800 40 50 40 quoting reference 830 of 8 July.
Alternatively, you can call Crimestoppers anonymously on 0800 555 111.
