The Korean Railway Workers' Union went on an indefinite strike as of 9 AM Wednesday.
Substitute workers were in place to operate high-speed KTX and commuter trains, but passengers are already being affected.
KTX trains are expected to run at around 69 percent of usual levels, SRT trains will run at around 78 percent.
Commuter trains are expected to operate at some 60 percent and freight trains 31 percent.
During off-peak hours, subways in Seoul are expected to run at about 82 percent of usual levels.
Korean Railway Workers' Union on first indefinite strike in three years
This is the union's first indefinite strike in three years.
There was transport chaos in South Korea in late 2016 when they walked off the job for 74 days.
The union and KORAIL have held numerous talks since May, but couldn't narrow their differences.
KORAIL workers and management last held talks on Tuesday, but again failed to reach a compromise on wages and employee numbers.
"The union workers are currently demanding a four percent increase in basic salary, and an additional four,and-six-hundred workers."
KORAIL has been operating a 24-hour emergency transportation unit since Monday.
The CEO of KORAIL, Son Byung-seok said on Wednesday that KORAIL will do best to ensure passenger safety,… and resolve the issue with the union through dialogue.
Transport Ministry to mobilize alternative means of transportation
The Ministry of Land, Infrastructure, and Transport has also mobilized alternative means of transportation in consultation with bus operators and provincial offices.
Messages were sent to customers who have booked trains that have been canceled as a result of the strike.
Affected customers will get a full refund on the price of their train ticket.
Kim Jae-hee, Arirang News.