Drivers of subway trains in Seoul often flick off a safety system that prevents crashes because it’s buggy, which may have been a contributing factor to the rear-ending on Seoul Metro line No. 2 Friday that injured more than 200 passengers.

Several lines, including line No. 2, have automatic train-stopping systems (ATS). Developed in Japan in the 1980s and 1990s, the system is designed to stop a subway train within five minutes if it does not halt at a “stop” signal in order to prevent crashes, according to Seoul Metro.

Officials of both the Seoul Metro and Korea Railroad (Korail) told the JoongAng Ilbo yesterday that drivers frequently turn off the ATS because it has frequent errors.

An eastbound train on Seoul Metro line No. 2 leaving Sangwangsimni Station in Seongdong District, eastern Seoul, was rear-ended Friday by another train that failed to stop at a signal.

Whether or not the driver of the train at fault turned off his ATS system may never be known.

“The ATS system has no facility to record whether drivers used it or not,” an official said. “There is no way for the subway control center to confirm whether drivers turned it off or not. It is also hard to figure it out later.”

When asked why drivers turn off the ATS, a railroad analyst said, “It’s because the ATS system, which is old analog technology, conflicts with the automatic train operation [ATO], which is new and digital. They are both used in operating Seoul Metro line No. 2.”

According to Seoul Metro, the trains on subway line No. 2 are being changed to the newly adopted ATO system.

Currently, 38 trains are using the new system, which was developed by Siemens and other companies in the 2000s.

There are still 50 trains using the old system, which was developed in Japan in the 1980s and ’90s.

Unlike the ATS system, a driver can’t turn off the new ATO system, officials said.

Due to mechanical conflicts between the two systems, errors were common on subway line No. 2, an official in charge of subway train control for Seoul Metro said.

“In order to use both the ATS and ATO at the same time, we need another system to connect them,” the official said.

“Including the interface system, subway line No. 2 is using three systems at the same time, so it frequently suffers errors due to the collision among the systems.”

A Seoul police official said they are investigating whether the driver who smashed his train turned off the system or not.

“It is not known yet whether this accident occurred because of a signaling malfunctions of the subway line or because the driver turned off the ATS system,” the police official said.

Seoul Metro officially denied the allegation that drivers flicked off the ATS in the past.

“There was no case in which a driver turned off the ATS system without an approval from the central control center,” a Seoul Metro official said. “Sometimes, if something happens, a driver calls the control center and is allowed to turn off the ATS and then drives the train very slowly.”

Meanwhile, the Seoul Metropolitan Police Agency released an interim report on the ongoing probe into Friday’s accident, adding more details about the faulty signal system.

According to the police, a Seoul Metro official found an error on the signal system at around 1:30 a.m. on Friday, about 14 hours before the two subway trains crashed.

But the official didn’t do anything to fix the error, the police said, based on questioning of Seoul Metro officials and an on-site investigation at the accident site.

The driver who operated the train in the rear, identified by the surname Eom, said he passed two “go” signals after he departed from Sindang Station.

Following those two signals, he saw a “stop” signal after he rounded a curve. He said he stepped on the brake pedal, but failed to stop the train immediately, only to crash into the back of the train ahead.

Police said the driver of the train in front, surnamed Park, said he did not report to the central control center that he departed from Sindang Station 90 seconds later than scheduled. He said the delay was because some doors did not close.

BY KANG IN-SIK, KIM HEE-JIN [heejin@joongang.co.kr]