RMT conductors plan to walk out for 24 hours from midnight in a row over their role on trains.
Southern said more than 70% of its trains would run - 10% more than on previous RMT strike days.
The union accused Southern of intimidating staff in a letter ahead of the strike, but it said the action would still go ahead.
Train drivers' union Aslef suspended a three-day strike planned for this week in return for talks with Southern starting on Monday, brokered by the TUC.
The RMT has not been given access to those talks.
General secretary Mick Cash said: "Southern should be engaging the RMT in the same way it has engaged with the TUC and the government. That is the way forward."
The union's 12 driver members still plan to take action on 24, 25 and 27 January.
The RMT said Southern's letter to staff had been "intended to cause doubt and confusion ahead of the strike".
The rail operator said: "Before every strike, we always write to conductors setting out the implications of not turning up to work; for example, in terms of loss of pay.
"We also feel we have an obligation to set out to staff how pointless these strikes are now and the continued misery their action causes our passengers."
Southern said it had put drivers in sole control of train operation, including closing the doors, on nearly 80% of routes, and had transferred many conductors to the new on-board supervisor role.
Angie Doll, Southern's passenger services director, said an extra 200 trains would run on Monday and several routes would have their first service on the strike day.
Talks between Southern and Aslef will resume on Monday.
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