An investigation has been launched after a woman reported that a man allegedly performed a sex act while sitting opposite her on a train.
British Transport Police said the incident happened shortly after midnight on June 16 on a Lioness Line service travelling from London Euston to Watford Junction.
Officers said the man sat opposite the woman and stared at her throughout the journey while masturbating.
Now, British Transport Police has released a CCTV image of a man they would like to speak to as part of their enquiries.
Police believe the man pictured may have information which could help with their investigation.
Anyone who recognises him is asked to contact British Transport Police by texting 61016 or by calling 0800 40 50 40, quoting reference 17 of 16 June.
Information can also be given to Crimestoppers anonymously on 0800 555 111.