A man allegedly sexually assaulted a female on board a train between Lincoln and Gainsborough.
British Transport Police detectives are investigating a report of a sexual assault on board a train on November 30.
A man boarded the Lincoln to Sheffield train with a group of friends shortly before 7.30pm on that day.
He sat next to a female at a table and it is reported he sexually assaulted her in between Lincoln and Gainsborough.
It is believed the man left the train at Sheffield.
British Transport Police released a CCTV image on December 11 of a man they believe may have information which could help with their investigation.
Anyone with information, or who witnessed the incident or recognises the pictured man, should contact BTP by texting 61016 or calling 0800 40 50 40 quoting reference 655 of November 30.
It can also be reported through the independent charity Crimestoppers anonymously on 0800 555 111.