The boy was told he had the ‘skin colour of a dog’ on a train between Hassocks and Preston Park on September 24, BTP said.
Detectives are now investigating the hate crime and believe the person shown in the CCTV image may have information which could help them investigate.
A spokesman said: “At approximately 3.15pm on September 24, a group of friends were on board a train travelling between Hassocks and Preston Park.
“During the journey they were approached by a group of men who began to harass them and later verbally abuse them.
“The abuse was racially aggravated and they also threated the group of friends with violence.
“One of the men then drank from a bottle of alcohol before spitting it at one of the friends.”
DC Imogen Sweeney from BTP, added: “Hate crime will never be tolerated on the rail network and we have a number of enquiries underway to identify those responsible for this abuse.
“I would encourage anyone with information to pick up the phone and call or send us a text message with details. Your information might prove essential in our investigation.”
Anyone with information can contact BTP by sending a text to 61016 or call 0800 40 50 40 quoting reference 404 of 24/09/2018.
Alternatively, people can call Crimestoppers anonymously on 0800 555 111.