A malfunction in the water pumping system at Bishan station caused water to collect in the MRT tunnel, leading to train service disruption along the affected stretch of North-South Line on Saturday that lasted close to 21 hours. The Land Transport Authority (LTA) said that preliminary investigations showed that water had entered the tunnel through the portal opening near Bishan MRT station, where above ground rail tracks transit underground, after a torrential downpour.

Under normal circumstances, rainwater runoff accumulated in the adjacent storm water sump pit would have been siphoned off by a system of pumps.

"Yesterday, the water pumping system malfunctioned, resulting in rainwater overflowing from the storm water sump pit into the tunnel opening and accumulating at the lowest point of the tunnel, between Bishan and Braddell stations," said an LTA spokesman.

SMRT immediately deactivated the track-side power supply as a safety measure. The pump system has since been repaired. Checks have also been conducted on other tunnel portal pump systems located throughout the North-South and East-West lines, added the LTA spokesman.

Detailed investigations into the cause of the disruption are ongoing. The disruption affected thousands of commuters travelling on a large stretch of the North-South Line from Ang Mo Kio to Marina South Pier for more than four hours on Saturday night. The disruption was also made worse by another incident, a track-side fire between Marina Bay and Raffles Place stations. LTA said that the fire was unrelated to the earlier incident, and it is investigating its cause.

While much of the train service was eventually restored on Saturday, the two-way service between Newton and Ang Mo Kio resumed only on Sunday at about 1:50pm. LTA said that officers from the Singapore Civil Defence Force and national water agency PUB worked through the night alongside officers from the LTA and SMRT officers to clear the water, with SBS Transit also on standby to support the recovery efforts.