Officers say the incident took place at around 7.45pm on Wednesday, December 31, 2025, on a train travelling from Staines to Clapham Junction.
According to investigators, a man walking down the aisle of the train carriage was sexually assaulted by another man who was seated.
Police believe the individual shown in the released image may have information that could assist with their enquiries and are urging anyone who recognises him to come forward.
A spokesperson for British Transport Police said officers are continuing to investigate the incident and are appealing to passengers or witnesses who may have been travelling on the service at the time.
Anyone who recognises the man pictured, or who has information about the incident, is asked to contact British Transport Police by:
