Commuters on the North-South (NSL) and East-West (EWL) MRT lines were affected by delays throughout yesterday morning.
SMRT, which operates both lines, told The Straits Times the delays were caused by track circuit faults.
Such faults, which are not the same as the more common track faults, arise because of ageing track circuits.
SMRT's vice-president for corporate information and communications, Mr Patrick Nathan, said in a statement that the faults resulted in trains having to travel at a slower speed for commuter safety.
"In the new signalling system that is currently being tested, the ageing track circuits will be replaced with a more advanced system that is more reliable as it is built with multiple redundancies for greater reliability," Mr Nathan added.
"After we renew the signalling system, faulty track circuits will no longer cause prolonged delays... We apologise for the inconvenience caused to affected passengers."
SMRT first tweeted about a fault at Khatib station on the NSL at 7.19am, when it warned commuters to expect a 10-minute delay from Bishan to Khatib. This was later revised to 20 minutes. Normal service resumed more than five hours later, at around 12.40pm.
Travel on the EWL from Paya Lebar to Simei was also affected by a similar fault at 8.35am, with commuters experiencing delays of up to 15 minutes. Normal service on the line resumed at around 11am.
Commuters affected by the train delays included bank executive Aaron Phang. The 26-year-old said he was 20 minutes late for work, adding that he regularly encounters such incidents on his commute on the NSL to work.
Another commuter, polytechnic student Celine Koh, 19, said she was not really affected because "the school understands if everybody is late", due to the incident's scale.
SMRT said last week that all-day trials of the new signalling system on the NSL will be carried out on Sundays, starting from April 16.
Trains are expected to run normally during the trials.
•Additional reporting by Ng Wei Kai