The gas leak at Tanjong Pagar MRT station on Monday afternoon was caused by an oil leak in a train's air-conditioning compressor.
Transport Minister Khaw Boon Wan told Parliament yesterday that the train was a first-generation model from Japanese manufacturer Kawasaki, with parts that are more than 20 years old.
The incident, he said, underscores the importance of maintenance - a call he has made previously in response to the regular train breakdowns over the past five years.
Likening older trains to an ageing human body, Mr Khaw said: "If you look after your body well from day one, when you are 40 years old... you probably can still maintain pretty good form.
"But still, a 40-year-old is not the same as a 14-year-old, and likewise, (it is) the same thing with trains."
Mr Khaw said preliminary investigations suggested that the train's air-con compressor had become overheated after the oil leaked.
This resulted in freon gas being released from the air-con system to the platform area, which initially sparked fears of a fire.
Mr Khaw was responding to Mr Sitoh Yih Pin (Potong Pasir), who asked for an update on the leak.
Freon gas is a common refrigerant used in air-conditioners. It is non-toxic and not flammable.
Mr Khaw said engineers will examine the compressor to check what had caused the oil, which is sealed within it, to leak. It was last checked about 10 days ago.
Videos posted on social media showed commuters rushing out of what looked like a smoke-filled train.
Operator SMRT later said passengers left the train, and its staff switched on the tunnel ventilation system to disperse the gas. The train was later withdrawn from service.
The gas leak has nothing to do with the cracks that formed underneath the carriages of several newer Kawasaki trains manufactured in China, said Mr Khaw.
The leak is the latest air-con-related incident on the MRT network. Earlier this month, The Straits Times reported two cases of ice shards falling from the air-con vents of China-made trains plying the Circle Line.
Last month, there were also reports of water streaming down from the air-con vents of an East-West Line train.