British Transport Police officers have released an image of a man they wish to speak to following an incident at London Bridge Underground Station.
The incident happened at 2pm on Friday 10 April.
A man approached the tail end of the platform when he saw another man at the far end of the platform shouting homophobic language and exposing himself.
The man re-dressed himself and boarded a southbound train.
PC Leslie Dunnett said: "I'd like to speak to the man in the CCTV image as I believe he may have vital information which could help with the investigation.
"People have the right to travel on the rail network without being made to feel uncomfortable or intimidated.
"Anyone with any information should contact British Transport Police on 0800 40 50 40 or text us on 61016 quoting crime reference number T-SUB/B2 of 17/11/15."