Members of the Rail Maritime and Transport union at South West Trains will stage walkouts on March 23 and 31 after voting by 5-1 in favour of action.
Drivers and guards at London's Waterloo station were balloted in support of Sarah Friday, who claims she was dismissed because of her union activities.
The RMT assistant general secretary, Bob Crow, said he believed Ms Friday was victimised because of her role as union health and safety representative at Waterloo.
Ms Friday said: "The company ... will have to reconsider in the light of this result."
A spokeswoman for South West Trains said: "We are obviously disappointed with the RMT announcement, particularly taking account of the low turnout for the vote."