Emergency brakes applied for an unknown reason rendered a Circle Line train inoperable near Marymount station during the morning peak period yesterday.
The Straits Times understands that the faulty train had to be pushed out of the way by another train.
The episode - just two days after a track fault disrupted service on the Bukit Panjang LRT for some six hours last Saturday morning - affected thousands of commuters on their way to work and school.
According to the Singapore Examinations and Assessment Board, a number of N-Level exams were held yesterday morning.
Circle Line operator SMRT first tweeted about the fault at 9.25am, warning of journeys taking 15 minutes longer. Later, it revised this to 30 minutes.
But National University of Singapore student J.R. Tan's journey from Bartley station to NUS took nearly an hour longer. After getting stuck in a train that was crawling and stopping, the second-year student got off at Botanic Gardens station and took a bus.
It was not the first time a train service was affected by emergency braking. Usually, a train will apply emergency braking if it senses a safety breach. Almost exactly a year ago, the Circle Line was beset with cases of trains applying emergency braking intermittently.
The 22-year-old said she missed two-thirds of a lecture. "When the train stalled at Caldecott, they announced that it was going to be a five-minute delay. Then 30 minutes later, they said it was going to be a 30-minute delay.
"If they had said 30 minutes from the start, I would have got off and taken the bus, and maybe made it in time for my lecture."
Engineer Bismarck Chang was more fortunate. The 39-year-old was on his way from his home in Bedok to work in Ang Mo Kio when the incident happened.
He managed to get out at Tai Seng station and hailed a cab.
He said he was disappointed that at the station, the staff there did not tell passengers what was happening.
Across the network, crowds packed station platforms.
Past 10am, SMRT tweeted to say service was recovering gradually.
It was not the first time a train service was affected by emergency braking. Usually, a train will apply emergency braking if it senses a safety breach.
Almost exactly a year ago, the Circle Line was beset with cases of trains applying emergency braking intermittently.
The long-drawn saga was initially attributed to an unidentified signal interference and the Land Transport Authority even considered equipping trains with an "electromagnetic shield" as a remedy.
The fault was eventually traced to a faulty train which was emitting rogue signals which affected other trains.
The discovery was made by external parties, including three engineers from the Defence Science and Technology Agency.
As at 5pm yesterday, SMRT was unable to say what triggered the emergency braking.