More dates have been announced for strike action on the Southern Rail network next month.
The industrial unrest comes after members of union Aslef voted to strike in protest at driver-only trains.
Members will walk out for 48 hours from December 13, for 24 hours on December 16, and between January 9-14.
The union has accused the company of wanting to impose changes, including the extension of driver-only trains, rather than reach an agreement.
The row is separate from the long-running dispute between Southern and the Rail, Maritime and Transport union over changes to the role of conductors, which has led to regular walkouts in recent months.
Aslef general secretary Mick Whelan said: ‘We have done our level best to try and reach a sensible, workable compromise with Southern in the interests of passengers as well as staff.
‘The Department for Transport’s fingerprints are all over this dispute. It’s as if the DfT is the ventriloquist and Southern the ventroliquist’s dummy.
’It doesn’t want to talk, it wants to bully. It doesn’t want to discuss, it wants to impose. Because it doesn’t care about passenger safety, only about profits for shareholders.’
Charles Horton, chief executive of Southern’s owners Govia Thameslink Railway, said: ‘It’s perfectly safe for the driver to have sole responsibility for the operation of a modern train, and that’s how a third of the trains up and down the country - with the full agreement and support of Aslef - already operate today.
‘The travelling public has already suffered months of misery and hardship as a result of the RMT’s pointless series of strikes.
‘We urge Aslef to get round the table with us to continue our talks and resolve their dispute without causing further unnecessary grief to passengers.’
Members of the RMT are due to stage a series of three-day walkouts on December 6, 22 and 31 in their long-running row over changes to the role of conductors.