Conductors on East Midlands Railway (EMR) are to stage a series of strikes in a dispute over pay and conditions.
More than 200 members of the Rail, Maritime and Transport union have said they will walk out on 16, 23 and 30 May.
The union said its members had strongly supported the action in a ballot.
EMR said it was "extremely disappointed" by the timing of the decision, just as services are due to increase.
RMT general secretary Mick Cash said: "RMT has been left with no choice but to call strike action.
"Despite clear demands from RMT, East Midlands Railway have refused to stop enforcing inferior contracts for some train guards which have been resisted for years and the patience of members has now reached its limit.
"The union remains available for talks to resolve this dispute."
Bosses at EMR said their latest offer included a reduction in working days and hours with no change to salary, the introduction of a new apprentice scheme and a new mentor scheme with a 4% salary increase.
In a statement, the company said it was "extremely disappointed RMT members have voted in favour of strike action".
Will Rogers, EMR managing director, said: "This is the wrong decision for the East Midlands and the towns, cities and communities we serve.
"This strike action comes at a time of national recovery, when the government is paying billions of pounds to keep trains running and protect jobs."
EMR added it was "confident in [its] contingency arrangements".