The victim, a 44-year-old man from Stratford, was left needing stitches after the horrific attack at Grays.
British Transport Police (BTP) detectives investigating the incident have released two pictures.
Officers, who appealed for information earlier this month, seized the onboard camera images of a man they are keen to speak to about the incident, which took place on Saturday, 28 December.
Detective Constable David Bishop, the investigating officer, said: “Whilst on board the 8.50pm Fenchurch Street to Shoebury service, the victim, a 44-year-old man from Stratford, began to engage in a conversation with another passenger.
“At first the conversation between the two men was amicable, with the victim sharing a drink with the man.
“The conversation turned hostile however and, as the victim stepped back to move away, the man used the glass he was holding to hit the victim on the back of the head.”
Both men left the carriage as the train arrived at Grays station, around 9.25pm, where the suspect ran off. Police were called and the victim later attended hospital where he was treated with stitches for the wound.
DC Bishop added: “Our enquiries have been ongoing and, after viewing CCTV from the area on onboard the train, we have today released a still of a man we are very keen to speak to.
“The CCTV images are very clear and I’d urge anyone who may recognise the man, or who may have witnessed what took place, to come forward.”
Anyone with information is asked to contact British Transport Police on 0800 40 50 40 quoting incident number LNA/B6 of 30/12/2013.
Information can also be passed to the independent charity Crimestoppers, anonymously on 0800 555 111.