A woman was pounced on by one of a group of men wearing Army shirts after the annual Army vs Navy rugby match at Twickenham, police said.
The group of four men - all wearing supporters tops - sat next to the 35-year-old woman after boarding a train at Twickenham station.
Police said one of the group then "grabbed the woman and started kissing her" in a "frightening" incident on board the 5.28pm train to Reading on May 9.
When the woman tried to pull away, the man is said to have held on to her and laughed as she "struggled to free herself from his grasp", according to detectives.
The woman left the train at Reading and reported the incident.
Police today released a CCTV image of a man they want to speak to in connection with the incident. He is also believed to have left the train at Reading.
PC Katherine Billingham said: "The man may have thought his actions were funny, but this was an uncomfortable and frightening experience for the woman.
“She made it clear that his advances were unwelcome, but he persisted anyway, restraining her in the process.
“This sort of unwanted sexual behaviour is not acceptable and we work hard to tackle it."
The Army vs Navy rugby union match is played annually between the senior teams of the Royal Navy and British Army.
May's game saw the Army triumph 38-18 in front of a capacity crowd of 81,116 at Twickenham.
Anyone who recognises the man is asked to call police on 0800 40 50 40, or text 61016, quoting reference SSUB/B3 of 02/07/2015, or contact Crimestoppers anonymously on 0800 555 111.
