The victim was travelling on the 8.19pm service on Sunday, February 18, when he asked a man to re-position as their leg was resting against him.
A British Transport Police spokesperson said: "The man then launched a torrent of abuse, threatening the victim and grabbing him around the throat, before pushing him towards the ground.
"The incident was reported by a number of passengers, who said they were frightened by the behaviour, but the victim has not yet reported this to police."
Officers have now released a picture of the man suspected to have carried out the attack and are appealing for anyone who knows him to come forward.
Anyone with information is asked to contact BTP on 0800 40 50 40 or text 61016, quoting the reference 166 of February 19.