Two men approached a 17-year-old girl on-board a train going to Cleethorpes and allegedly started making inappropriate sexual comments to her.
British Transport Police (BTP) officers are investigating the incident which happened on-board a service between Doncaster and Cleethorpes at around 6pm on Saturday, July 16.
One of the men is said to have then touched the victim inappropriately before asking her to go to a hotel with him.
A number of passengers witnessed the incident and intervened.
Both men left the service at Scunthorpe railway station.
Officers are appealing for witnesses and are keen to speak to anyone on-board the train who saw the incident.
Witnesses or anyone with information should contact BTP by taxing 61016 or calling 0800 40 50 40 quoting reference 722 of July 16.
Alternatively, information can be reported through the independent charity Crimestoppers anonymously on 0800 555 111.