The victim was asleep on a Southwestern Railway service from Surbiton to Basingstoke between 11.40pm on Saturday, June 14, and just after midnight into Sunday, June 15, when he suddenly felt water on him and woke.
After making a noise, he was threatened by another passenger, who warned he would “punch him” if he reacted again—before following through on the threat and launching a violent assault.
The man was punched several times in the unprovoked attack.
Fellow passengers stepped in to try and break it up.
The victim got off the train at Walton-on-Thames.
The suspect is believed to have left the service at Weybridge.
British Transport Police have now released a CCTV image of a man they want to speak to in connection with the assault.
They believe he may have information that could assist their investigation and are appealing to the public for help identifying him.
Anyone with information is urged to contact BTP by texting 61016 or calling 0800 40 50 40, quoting reference 04 of 15 June.Alternatively, contact Crimestoppers anonymously on 0800 555 111.
