Officers are investigating an assault on board a South Western Railway train from Surbiton to Basingstoke.
It happened between 11.40pm and just after midnight on Saturday, the 14th of June.
A man was asleep on the train when he had water poured over his feet, woke and made a noise.
Another man then threatened to punch him if he made the noise again.
He was then assaulted and punched repeatedly with passengers trying to intervene before the victim exited at Walton.
The man is believed to have left the train at Weybridge.
Officers believe the man in the image may have information that can help their investigation.
Anyone with information, who recognises the man or who may have seen the incident is asked to contact BTP by texting 61016, or by calling 0800 40 50 40 quoting reference number 04 of 15 June.
Alternatively, call Crimestoppers anonymously on 0800 555 111.