The 46-year-old victim was working on his laptop when he noticed a man staring at his screen.
He asked him not to look and as he went to move across the carriage, the attacker punched him in the head, before quickly making his escape.
The attack took place on a train between Clapham Junction, in south London, and Gatwick on 10 November.
The victim suffered bruising to his head and damage to his knee, which he injured during the assault.
Over two months later, British Transport Police has released a picture of a man wearing a black North Face jacket, who investigators say may have information that could help the investigation.
The man can be seen holding a Burger King cup
Anyone in any information regarding the incident is asked to contact British Transport Police on 0800 405040.