SINGAPORE - Service on the North-South Line is expected to be slow for the rest of Wednesday (Oct 25) as a track fault can only be fixed after operational hours.

A rubber insulator melted along the city-bound tracks of the MRT line early Wednesday morning, creating a gap between running rails.

For safety, transport operator SMRT has been running trains at lower speeds over a 150m stretch between Bishan and Ang Mo Kio stations.

This has resulted in a bunching up of service on the heavily-used line during the morning peak, which is expected to repeat during the evening peak.

The Straits Times understands the melting was caused by an electrical arcing - similar to the one which sparked a trackside fire between Marina Bay and Raffles Place stations on Oct 7, and which was caused by a build-up of debris.

Wednesday's incident came a day after a delay on the East-West Line, which SMRT said was caused by a software glitch.

On its Facebook page, SMRT said its engineers "are monitoring the situation closely". It added that free bus services between Yishun and Toa Payoh are available.

"Bridging bus services will also serve commuters from Yishun towards Toa Payoh. Alternative bus travel options are available at http://bit.ly/2rMan0V," it added. "You can also consider taking the North-South Line towards Jurong East station and transfer to the East-West Line. We do apologise for the inconvenience this is causing you."

Just last week, SMRT apologised for an unprecedented flooding of the North-South Line which disrupted service for 20 hours on Oct 7 and 8.

Mr Sitoh Yih Pin, chairman of the Government Parliamentary Committee for Transport, said: "It is extremely unfortunate that there have been train delays on the East-West Line and North-South Line in the past two days. It is understandable that commuters are frustrated by the delays as they have been inconvenienced again.

"Our train operators must work hard to rectify any faults expediently and ensure that repeat incidents are mitigated. If there is a systemic issue to be addressed, it should be addressed."

The two faults this week took place in the midst of O Level examinations.