Sadang Station in southern Seoul is crowded with passengers early Monday, as the northbound track of Seoul Subway Line 4 was partially suspended during rush hour due to a power outage. (Yonhap News)

Seoul Subway Line 4 was partially suspended by a power outage during rush hour on Monday morning, the first day of the subway’s full operation since the union’s longest strike ended.

The power supply was suddenly cut on the northbound tracks between Geumjeong Station and Gwacheon Government Complex Station in Gyeonggi Province at 5:30 a.m., affecting the northbound route to Sadang Station in Seoul for nearly two hours.

Line 4, jointly operated by Seoul Metro and the Korea Railroad Corp., connects southwestern and northeastern Seoul through the center of the capital.

KORAIL, which has exclusive control over the problematic section, took emergency measures and resumed rail service at 7:40 a.m.

“The exact cause of the power outage is still under investigation, but it is assumed to be the poorly insulated power lines,” a KORAIL official told The Korea Herald.

The train operation was delayed more than 20 minutes each way as the northbound trains had to share the southbound lane.

“It took over 30 minutes longer than usual to get to work because I had to take another subway and transfer to the bus. It was more crowded than other time,” said Jang Hyeok-jun, 27, who commutes from Geumjeong Station to Seoul City Hall.

It was the first day that the metropolitan subways started the full operation after the KORAIL’s labor union ended the 22-day strike late last month.

Trains operated at only about 85 percent of normal capacity during the strike.

The full operation of the KTX and freight trains is expected to start from Tuesday next week.

By Lee Hyun-jeong (rene@heraldcorp.com)