SINGAPORE - A track fault caused city-bound trains along a 150m stretch of the North-South Line (NSL) between Ang Mo Kio and Bishan stations to run slower for much of Wednesday (Oct 25) morning.

Train operator SMRT, in a tweet before the start of service at 5am, warned commuters to expect a delay of 20 minutes from Yishun to Toa Payoh stations. This was later revised to 10 minutes at 8.05am.

Normal service resumed at 10.13am. The fault was not linked to the new signalling project.

[NSL]: Due to a track fault, pls add 20mins train travel time from #Yishun to #ToaPayoh SouthBound. (Not linked to new signalling project). — SMRT Corporation (@SMRT_Singapore) October 24, 2017

In its last update on Facebook at 8.22am, SMRT explained that since the start of service on Wednesday, trains had to travel at reduced speeds for safety reasons.

"The speed restriction has been imposed over a 150-metre stretch of track between Ang Mo Kio and Bishan stations, in the direction of the city. Our engineers are monitoring the situation closely," it added.

The Straits Times understands that the fault occurred near Bishan swimming complex. Electrical arcing had caused the rubber insulator between the running rail to melt, resulting in a gap in the track.

ST understands that this is similar to the fault that caused a fire on the same line on Oct 7 between Raffles Place and Marina Bay stations.

Mr Terence Ng, who is in his 40s, tweeted a photo at 7.14am of what appeared to be a stalled train at Bishan station. SMRT personnel were spotted walking on the tracks.

"I noticed the staff walking on the track as I was on the overhead bridge opposite Block 216 Bishan Street 22," Mr Ng told ST. He added that he saw plenty of bridging buses while walking to Bishan station.

Train stalls at Bishan NSL towards Marina Bay. Track fault? #smrt pic.twitter.com/yZtV1Hqq2d — terencewu (@wuqinghe) October 24, 2017

SMRT had earlier said commuters should expect congestion during the morning peak at stations from Yishun to Ang Mo Kio due to the fault. It deployed additional staff at the stations.

Bus services from Yishun to Toa Payoh have been made free, while bridging services are also available.

Commuters heading to the city have been advised to take the North-South Line towards Jurong East station and transfer to the East-West Line.

SMRT's engineering teams are planning to work on the affected part of the track after the morning peak.

The latest incident comes just a day after commuters experienced congestion on the East-West Line during morning peak on Tuesday.

Trains on the line also moved at slower speeds as it encountered a "software fault on the legacy signalling system".