Passengers travelling on the North-South Line between Kranji and Bukit Gombak MRT stations during rush hour yesterday morning saw their journeys delayed significantly, after a signalling fault hit the line.

Train service from both directions along the route spanning four stations - Kranji, Yew Tee, Choa Chu Kang and Bukit Gombak - were stopped for about 45 minutes, starting at 8am.

Any delay of more than 30 minutes is regarded as a major delay.

In a Facebook post an hour after the fault was resolved, SMRT said its engineers were on site to resolve the issue. It also activated free bus and bridging bus services.

"Commuters in the affected trains safely disembarked at the stations," added SMRT. "We are sorry to have affected your morning commute."

Pictures posted online after the fault occurred appeared to show large crowds waiting for bridging buses at Choa Chu Kang bus interchange and Kranji MRT station.

Several commuters said on social media that they were on the trains between the affected stations when the fault happened. Some reported that the trains stalled for some time before they headed back to the previous MRT stations.

Major disruptions have become rarer in recent years amid heavy investment and effort poured into improving rail reliability. In August, Transport Minister Khaw Boon Wan lauded operators, regulators and suppliers for efforts in the area.

Figures last month showed the rail network was well on track in terms of reliability. A benchmark measurement of rail reliability - called mean kilometres between failure, or MKBF - crossed the one million train-km mark for the first nine months of this year. In contrast, in 2011, delays and disruptions happened every 58,000 train-km.

According to SMRT, between January and October this year, there was only one service disruption of more than 30 minutes on the North-South Line. There were no such disruptions on the line last year, and six in 2017.

Associate Professor Andrew Ng, who teaches engineering modules such as railway signalling at the Singapore Institute of Technology, said a signalling fault could either be an isolated incident or a potential indicator of a bigger problem, depending on the root cause.

"A railway signalling system is a complex combination of hardware, software and communication networks," he added. "As such, signalling failures are inevitable, even with the relatively new signalling system like (in the) North-South Line."