The collective action by the unionized workers came as overnight negotiations with the railway operator fell through.
Around 30 to 70 percent of passenger trains, including the KTX bullet train, intercity subways and the Saemaeul and Mugungwha services, as well as cargo trains, are forecast to be affected by the strike that started at 9 a.m.
The transportation ministry plans to mobilize Korea Railroad Corp. staff and military forces to run trains to minimize passenger inconvenience during commuting hours.
They also plan to request the support of bus operators and provincial offices to run additional public transportation.
The latest strike follows a 72-hour walkout last month.
Unionized workers had pledged to hold another strike if their demands, including additional employment and a wage increase, were not met.
It is the first strike without a fixed schedule in three years, following a 74-day strike from September to December 2016. (Yonhap)