A man who was asleep on a train woke up to find another man sexually assaulting him.
The attack happened on a train from London Liverpool Street to Colchester, at around 7.30pm on March 4.
On Friday, the British Transport Police released a photo of a man they believe may be able to 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 number 325 of 5 March.
Alternatively you can call Crimestoppers anonymously on 0800 555 111.
It comes on the same day a search was launched for a different man who the BTP believe could help with an investigation into a 19-year-old woman being sexually assaulted.
She was on the Piccadilly line when a man who had been sitting next to her stood up and deliberately fell onto her.
He then sexually assaulted her, stood up and smiled at her, officers said.
This took place when the train reached Holborn station, at around 5.30pm on August 17 last year.
