The incident happened at Whiteabbey Railway station on Thursday September 27th, but details only emerged this week.
A PSNI spokesperson said: "At around 4:15pm, a girl in her mid-teens was approached by a male at the country-bound platform and was sexually assaulted.
"Police would like to speak to three young males - possibly schoolboys in uniform - who were reported to have been on the Larne side of the halt and may have witnessed the incident."
The incident has shocked many local people and politicians have called for witnesses to come forward with information.
Noreen McClelland, a Newtownabbey councillor, said: "This is a horrific thing to happen to a young girl. People who commit this type of attack are likely to do it again, so the person responsible must be caught as soon as possible. The police have put out an appeal for witnesses and I would urge anyone who may know anything which could help catch the attacker to come forward as soon as possible."
Democratic Unionist Assemblyman Alastair Ross said the person responsible needs to be taken off the streets.
The East Antrim MLA said: "It is always disturbing to hear of these kinds of attacks, particularly when the victims are young girls, and it is important that whoever is responsible is caught and taken off the streets."
He added: "The fact that this attack happened during daylight hours will cause even more concern for those living in the Whiteabbey area, particularly among young women and parents of young girls. We always advise young people to stay with friends when out and about at night, but something is really wrong in society when a young girl is attacked in such a cowardly and repulsive fashion before tea time on a Thursday afternoon.
"I would appeal to anyone who has any information whatsoever to contact the PSNI immediately and help to remove this dangerous individual from the community, and for young women in the area to be extra vigilant when out on their own."
Anyone who saw the incident or was in the area on September 27 can contact police on 0845 600 8000. Anonymous information can also be passed to Crimestoppers on 0800 555 111.