The incident occurred around 8:50pm, lasting around half an hour, on Sunday 13 May.
British Transport Police said that one young boy, believed to be aged 12 or 13, began shouting racially offensive and sexually explicit comments at a teenage girl.
He was accompanied by a group of at least four older boys aged around 14-17.
The tirade of abuse happened while the train was stopped at Balloch station.
The train driver refused to depart the station until the boy got off the train. The boy complied and left with another boy from the group.
Aged 12-13, the boy is described as white, around 4ft6 with short fair hair.
Anyone who saw what happened or who recognises the description is asked to contact BTP by texting 61016 or calling 0800 40 50 40, quoting 474 of 22 May.
Info can also be passed via Crimestoppers anonymously on 0800 555 111.