A man is alleged to have indecently touched himself while watching pornography on a Lincoln train.
British Transport Police released CCTV images of a man they believe could help with their investigation.
The incident was reported at 9.35am on Wednesday, December 19 between Lincoln and Sleaford.
A man is believed to have exposed himself and indecently touched himself while watching pornography.
Anyone who recognises the man pictured or has information about the incident is being urged to call 0800 40 50 40, quoting 115 of December 19.
Update: Police have now been in contact with the man they wanted to speak to. The investigation continues.