Train services on SBS Transit's North-East Line were disrupted for one hour and 40 minutes during the morning peak period yesterday.
The incident was triggered by a power fault, which also caused a blackout across several stations along the line.
The latest incident came a month after another glitch brought the system down for more than two hours during the morning rush hour, making it the fifth major disruption for the 12-year-old line this year.
SBS, a subsidiary of transport giant ComfortDelGro, said there was a power trip at 7.41am that cut electricity supply to its substation at Buangkok station.
This substation distributes power to station amenities such as air-conditioning and lights, as well as traction for the trains.
The operator said it managed to restore power progressively from 8.02am, and that trains started moving from 8.11am.
However, it was not until 9.30am that service frequencies were back to normal.
"We apologise to all passengers for the distress and inconvenience caused," spokesman Tammy Tan said. "We are investigating the cause of the fault."
Bank employee Wee Huixian, 29, said she took 11/2 hours to get to work from Buangkok to Marina Bay - double her usual 45 minutes.
She said the station was stuffy and "people were quite lost" as there were only two staff members present and they could not tell commuters when service would resume.
Public relations consultant Paula Robinson, 35, had to cancel a work meeting because of the delay.
When trains started moving again, she said, "people were pushing to get on".
She also saw a man get his foot stuck between a train's doors. She said: "A gentleman and I pulled the doors open to free his leg. Scary moment, because the doors did not want to open, and we barely got them open enough to free his foot before they closed again."
Public servant Calvin Tan, 28, usually catches a bus from Kovan to Serangoon station to take a Circle Line train.
However, as all the buses were too crowded to board, he walked to Serangoon, more than a kilometre away.
"It usually takes 35 minutes for me to get to work," he said. "But today, I took 1 1/2 hours."
Teacher Laremy Lee, 32, said his ride from Serangoon to Dhoby Ghaut "usually takes 20 minutes tops".
"This time round, it took about 40 minutes," he said, adding that because of the delay, he missed the early-bird free ride.
However, he said, it was his first delay since he started taking public transport regularly 1 1/2 years ago. "It's part and parcel of the experience," he said resignedly.
Mr Seah Kian Peng, an MP for Marine Parade GRC, which the North-East Line serves, said the frequency of disruptions on the relatively short line is "not a good sign".
"The causes of failure need to be looked into with greater intensity," he said. "At the same time, we need to work on recovery standard operating procedures."