Southern Rail drivers will now strike for three days in late January, in addition to the three-day walkout next week.
The fresh wave of strikes will take place between January 24 and 27, the Aslef drivers' union has announced.
Next week's walkout will begin on Tuesday (January 10) and continue on January 11 and 13.
The strike was initially scheduled to last three days but has been halved, meaning beleaguered commuters in London and the south-east will be able to travel by train on Monday January 9 and Thursday January 12.
The union said next week's action was reduced due to concerns over its impact on the public, according to The Guardian.
The latest wave of industrial action is just part of a long-running debate between unions and Southern Rail over the role of conductors. Strike action began last February and has continued sporadically since then, prompting widespread criticism for all parties involved in the dispute.