British Transport Police have released CCTV images following a report of indecent exposure on board a train as it pulled into Derby station.
The incident happened at around 7.30pm on Saturday June 22.
A man is reported to have exposed himself to a woman before promptly leaving the service at Derby.
The train route was the 6.02pm London St Pancras to Sheffield train.
Officers believe the man in the images may have information that could help their investigation.
Anyone with information is asked to contact BTP by texting 61016 or by calling 0800 40 50 40 and quoting reference 579 of 22/06/19.