SINGAPORE - The delays that plagued the North-South (NSL) and Downtown (DTL) Lines on Friday morning (Aug 18) were caused by two separate and unrelated faults, said the Land Transport Authority (LTA) in a statement.

Preliminary findings indicate that the NSL fault may have been caused by a defective train transmitting erroneous signals.

This caused trains to move at a slower speed for safety reasons. Reduced train frequencies led to congestion at NSL stations. The defective train has since been pulled from service while engineers conduct further investigations into the cause of the fault.

LTA added that on the DTL, a fault in the line's supervision system prevented trains from being automatically launched from the depot.

Subsequent trains were then manually driven to the main line at a lower speed, leading to delays. The fault has since been rectified but further investigations are ongoing.

The problems on the NSL, which was also crippled on Thursday evening due to a fault on its brand new signalling system, prompted operator SMRT to issue an early warning ahead of Friday evening's peak hour to tell commuters to expect another system glitch.

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It said in a Facebook post: "Commuters who are planning to travel on the North-South Line (NSL) this evening are advised to plan their journeys ahead.

"Some additional train travelling time may be necessary."

It noted, however, that trains on the NSL have been "running smoothly since this morning" - the three-hour morning peak disruption notwithstanding.

"Our engineers continue to monitor the system very closely," it added. "Our staff will be on hand to assist commuters."