Two faults on the North-South Line yesterday morning led to delays for commuters during the morning rush hour.
The latest delays came just 12 hours or so after Wednesday night's two-hour disruption.
While Wednesday's delay was due to the testing of the new signalling system, yesterday's delays were caused by two separate incidents.
At 6.40am a train encountered a signalling fault at Woodlands.
"The train was driven manually at a slower speed towards Jurong East station, where commuters were safely disembarked," said SMRT vice-president of corporate communication Patrick Nathan, adding that the train had been withdrawn from service for further checks.
At 8.20am a faulty platform screen door at Bishan station delayed a train travelling southwards to Marina South Pier.
Commuters, who had to endure delays both on Tuesday and Wednesday, took to social media to vent their frustrations.
Twitter users complained of delays at Sembawang, Kranji and Yew Tee stations, among other locations.
One user said it took him close to 30 minutes to get from Yishun to Bishan, which are four stations apart.
Mr Peter Ooi, a 23-year-old student, said he entered Ang Mo Kio station at 8.20am to take the train when he heard multiple announcements that there was overcrowding at the platform.
He also noticed that there were many SMRT service staff telling people to line up and trying to organise the crowd. He decided to take the bus, which extended his travel time by 30 to 45 minutes.
Mr Nathan said: "The cumulative effect of these two faults on the same line during the morning peak resulted in longer waiting times for commuters. In-train and station announcements were made to keep commuters updated. We apologise for the inconvenience caused to affected commuters."
There were, however, no announcements on SMRT's Twitter or Facebook pages yesterday morning.
Commuters have had to face a number of delays over the last three months, since the Land Transport Authority (LTA) and SMRT started testing a new signalling system with the intention of eventually switching over to the system which will allow trains to run at shorter intervals.
Experts have said that it is sometimes necessary to conduct system tests during passenger service hours as the presence of passengers will affect how a train operates.
For example, the commuters' weight will affect the total mass of the train, which in turn will affect its speed control.
In a joint statement earlier this week, SMRT and LTA warned commuters of delays in the coming weeks.
They said that in the coming weeks, they would be conducting intensive performance checks and deploying new software to improve the system.
"During this period, commuters may continue to experience some delay when travelling on the North-South Line, if new issues emerge," said the statement.
LTA and SMRT said they "sought the continued understanding and patience of commuters".