At around 6pm on Saturday, July 16, two men approached a 17-year-old girl on-board the service and started making inappropriate sexual comments to her.
One of the men 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 keen to speak to anyone on-board the train who witnessed the incident as they may have information which could be crucial to the investigation.
Witnesses, or anyone with information, can contact BTP by texting 61016 or calling 0800 40 50 40 quoting reference 722 of 16 July.
Alternatively, you can call Crimestoppers on 0800 555 111.
*The British Transport Police’s mission is to help the millions of people who use the railways of England, Wales and Scotland get home safely and on time.