Several passengers commuting on a subway line connecting Seoul with its western satellite city of Gimpo expressed concern after they detected a burning smell on Wednesday.

The subway lines’ operator, Seoul Metro, said it had received a total of three complaints of a burning smell between Pungmu Station and Gimpo International Airport Station at around 7:50 a.m.

The operator said it immediately launched an investigation into the matter.

A passenger told local media that “a burning rubber smell started to engulf the train” and that they had detected a similar scent the day before.

The train’s operator made an announcement early Wednesday, reassuring passengers not to panic.

The complaints filed by passengers on Wednesday come not even a week after a similar incident happened on the subway line on Nov. 28, where 11 passengers complained of a burning smell. The operator explained that the smell was caused by damage to the subway’s brake system and said it had worked to fix the problem.

“We have resumed operations following the replacement of our brake system and a detailed inspection. We pledge to do the same for our latest incident,” a Seoul Metro spokesperson said in a statement.

The Gimpo Goldline has been the subject of scrutiny since the start of its operation in 2019 due to issues of overcrowding.

According to the Gimpo City Government, a total of 135 passengers were subjected to emergency health treatment caused by morning rush hour crowds from Sept. 4 to Nov. 24.

Seoul Metro recently decided to give up on its role as the subway line’s operator in September 2024, without citing a specific reason behind its decision. The operator has been in charge of the subway line since September 2019.

Seoul Metro says it plans to receive applications from various railway operators on Jan. 10 and make a decision on its replacement as of the end of March. The new 5-year contract is worth 150 billion won ($114 million). Five local public and private railway operators have submitted their offer as of the end of November.

Earlier this year, Transport Minister Won Hee-ryong presided over an emergency meeting to address overcrowding mitigation measures for the Gimpo Goldline.

Some Seoul Metro officials were deployed to Gimpo Goldline stations to manage overcrowding on the platforms and trains. Their main job was to restrict the number of people from boarding on trains to prevent carriages from becoming overcrowded. The Seoul Metropolitan Government also increased the frequency of buses on routes linking Gimpo to downtown Seoul and considered introducing an amphibious bus service between Gimpo and docks along the Han River.