Mr Jith Joseph had just boarded the train at Serangoon MRT station on the Circle Line when he noticed a woman pushing a pram into the carriage as the train doors were closing.
The doors closed shut right after the pram entered the carriage, leaving the woman in her 30s stranded on the platform in shock.
She banged on the train doors as it pulled away, distraught at being separated from her baby daughter, who is about a year old.
Quick-thinking Mr Joseph, 32, realised the child had no other family members on the train and alerted passengers that they had to help.
At the next station, Lorong Chuan, he asked two women to get off the train and stay with the infant on the platform, in case the mother arrived on the next train. He then hopped on a train and headed back to Serangoon.
All this happened on Saturday last week. The marketing manager, a new father, said he felt the mother's pain as his own daughter, Isabel, had been born in June. "I just wanted to help," he said. "I knew what she had to be going through."
Back at Serangoon station, the frantic mother had approached SMRT service ambassador Norlinda Ahmad, 44, for help.
Ms Norlinda said the woman was in tears: "She was shouting that her baby was in the train, that she was scared she would not see her daughter again."
When Mr Joseph arrived at Serangoon, he found the mum at the station control. "She was hysterical and kept blaming herself."
After reassuring her that her child was safe, Mr Joseph and Ms Norlinda accompanied her to Lorong Chuan. When the mother saw the pram, she ran to it and took her daughter in her arms. Said Mr Joseph: "The girl broke out into a smile when she saw her mother, who started crying again."
The scene also moved Ms Norlinda to tears. Mr Joseph estimates mother and daughter were separated for 15 to 20 minutes, which "must have felt like an eternity".
Good deed done, he left for home while Ms Norlinda stayed with the mother for about 30 minutes.
He said that after the mother thanked him, she told him that she was worried because she did not know if anyone would help her.
He recalled that although the train was not crowded, no one else reacted right away. "I really hope that if I wasn't there, someone else would have stepped up to help," he added.
He drew a round of cheers from his friends after he described the incident on Facebook.
SMRT director for corporate marketing and communications Alina Boey said such cases happen about once a month on the Circle Line. She advised parents in a similar position to stay calm and inform station staff who will notify other stations to look for the missing child.
"The most important thing is for the parent to stay put at one station, inform the staff and let them do the tracking," she said.
Public Transport Council chairman Gerard Ee advised commuters not to rush onto a train when they hear the doors beeping.
He added that commuters have to look out for one another too. "It's a collaborative effort, and not something that you just leave to the train operator to do."