A Essex ticket inspector suffered a broken jaw and cheek bones after a violent attack from a passenger who became aggressive after being challenged about his child's ticket.
The passenger even tried to throw the man from the train before jumping off himself.
British Transport Police (BTP) has now released CCTV images of the man they want to trace after the serious assault on an Essex train.
The incident occurred on the morning of Thursday, 12 March, on board a Southend-bound service as it travelled between Grays and Tilbury Town.
Train operator C2C has now offered a £500 reward for information leading to the successful arrest and prosecution of the person responsible.
Detective Constable Rod Harbron, of BTP's workplace violent unit, said: "At about 11am, a team of four rail staff boarded a train at Barking station and began to check tickets.
"When a passenger was asked to show his ticket, he handed over a child ticket. When challenged about his age, he became abusive.
"Moments later, the man punched the staff member in the face – breaking his cheek and jaw bone. During a violent struggle, the other three members of staff were also physically assaulted by the man.
BTP officers met the train at the next station and conducted a search of the area but nobody was found. An ambulance took the victim, who is 28 and from Essex, to Basildon University Hospital for treatment.
DC Harbron added: "This was a violent attack on four members of staff who were just trying to do their job. The initial unprovoked punch left the victim with broken jaw and cheek bones. He has understandably been left shaken by what took place and has not been able to return to work since the incident. Violence of this kind is absolutely unacceptable and we are doing all we can to catch the man responsible."
Officers are now appealing for the public's help identifying a man they want to speak to.
DC Harbron said: "Who is the man pictured? We need to speak to him urgently about what happened. If you recognise him, or have any other information, please get in touch with BTP."
C2C Security Manager Iain Palmer said: "Our employees are working on behalf of all passengers and they should be able to work without fear of being assaulted. That's why we're offering a £500 reward if you can help the police to catch these attackers."
Anyone with information is asked to contact British Transport Police on 0800 40 50 40, or text 61016, quoting reference E/B2 of 13/05/2015. Information can also be passed anonymously to independent charity Crimestoppers on 0800 555 111.
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