A KTX high-speed train derailed five minutes after leaving Gangneung Station in Gangwon Province en route to Seoul on Saturday morning.
There were no casualties since the train was doing less than 100 km/h when it derailed, but the damage could have been catastrophic if it had been near its top speed of 250 km/h.
This was only the second time a KTX derailed since services began in 2004. The last time was in 2011.
The Ministry of Land, Infrastructure and Transport and KORAIL said Sunday that the accident appears to have been caused by a malfunctioning train track switch box.
The switch box signals trains to proceed when the track is clear or halt when there are obstacles. But it appears to have sent the wrong signal, causing the train to enter the tracks even though there was a problem.
A series of smaller accidents have occurred recently. Last month there were six in the course of just one week. On Nov. 19 a KTX train crashed into an excavator as it approached Seoul Station, injuring three workers, while a power failure at Oseong Station the next day caused 129 high-speed trains to be grounded.
Critics say the ministry and KORAIL are more focused on inter-Korean railway projects than on railway safety here. KORAIL president Oh Young-sik and Transport Minister Kim Hyun-mee are politicians without expertise in railway operations.
One former ministry official in charge of railway operations said, "Safety should come first, but perhaps problems are happening because administrators are focused on other things."
The Gangneung-Seoul route returned to normal operation on Monday morning after two days of repairs.
A KTX train lies derailed near Gangneung Station in Gangwon Province on Saturday. /Yonhap