More than 800 rail workers at train operator One will conduct a 24-hour strike on 15 and 17 September, in support of a sacked colleague.
Members of the Rail Maritime and Transport (RMT) union backed strike action by 300 votes to 59.
It follows the sacking of guard Paul Yarwood for gross misconduct. Yarwood was dismissed over an incident at Colchester North on 7 June involving an abusive fare evader who refused to stop smoking, assaulted a platform supervisor, and threatened a passenger and other staff.
One claims Yarwood confronted and headbutted the alleged fare evader, who is now facing a trial having been accused of threatening and abusive behaviour.
More than 100 guards, revenue-protection inspectors and retail staff in north Essex have already taken two days of action in defence of Yarwood.
Bob Crow, RMT general secretary, said: “It is astonishing that the company still seems to have its head buried in the sand, not least because of the huge public support that Paul has had from members of the public.
“Our members will be leafletting passengers during the strike to explain that the company’s bizarre stance has left us no option but to strike again – this time, right across the company,” Crow added.