Commuters on the East-West Line (EWL) experienced delays which lasted for about two hours during morning rush hour yesterday.
The delays were caused by two different faults - a train fault and a track circuit fault - along the same east-bound stretch from Joo Koon to Clementi stations.
SMRT first announced the train fault on Twitter at 8.41am, warning commuters to expect an additional 10 minutes of travel time.
At 9.13am, it revised the delay upwards to 20 minutes, attributing it to a track circuit fault.
Normal train service resumed at close to 11am.
Commuters were critical of SMRT for not being transparent in reporting the exact duration of delays, with several claiming that they had experienced waiting times of more than 30 minutes.
This is the third time in a week that the EWL has been hit by delays.
Train service was delayed for about 30 minutes last Wednesday between Jurong East and Joo Koon stations due to a train fault, while a signalling fault on Monday caused delays of up to 25 minutes between Queenstown and Bugis stations.
The rail operator and the Land Transport Authority (LTA) had warned in June that SMRT's testing of a new signalling system may cause potential delays on the EWL and North-South Line (NSL), two lines which see about two million passenger trips every day.
The SMRT said there was "no choice" but to conduct checks for the new signalling system throughout the day.
After the delays on Monday, SMRT Trains' CEO Lee Ling Wee said it would take years to implement the new signalling system if testing was limited to the engineering hours of between 1.30am and 4.30am.