British Transport Police has released this image of a man they wish to question regarding an incident on a train between East Croydon and Gatwick.
The incident happened at approximately 9.30pm on Wednesday 28 December 2016.
British Transport Police say in a statement that female passengers were sitting on the train when they noticed a man opposite them exposing himself and touching himself inappropriately.
Officers would like to speak to the man shown in the CCTV image as he may have information which could help with the investigation.
If you recognise him, call British Transport Police on 0800 40 50 40 or text 61016 quoting reference number 200 24/01/17.