Thousands of commuters were affected after a track circuit fault caused trains to slow down and stall intermittently on the North-South Line for two hours during the morning rush hour yesterday, The Straits Times reported.

Republic Polytechnic (RP) second-year student Brian Lee, 19, missed his exams as a result. He said the train he took stopped for 15 minutes between the Jurong East and Choa Chu Kang stations, and again for 25 minutes as it headed towards Kranji.

SMRT said the fault between the Woodlands and Admiralty stations resulted in trains slowing down in both directions from 7:21am yesterday. By the time service resumed two hours later, at around 9:25am, congestion had built up at several MRT stations.

At the Yishun and Ang Mo Kio stations, for example, passengers said the escalators were shut down temporarily to slow down the inflow of commuters after the platforms became crowded.

Free shuttle bus services were offered to commuters from Yew Tee to Toa Payoh and Ang Mo Kio. SMRT said in a Twitter post that commuters could expect journeys to be delayed for up to 25 minutes.

An RP spokesman said 'a small number' of students missed the exams. He said those who arrived within a 30-minute 'grace period' could still take the exams. Those arriving more than half an hour after the start time would be evaluated on a case-by-case basis, he added.

There have been at least 15 rail disruptions or delays since the start of the year, including those on the LRT system. The number of major breakdowns - those lasting more than half an hour - hit a four-year high of 12 last year.

The Land Transport Authority said it is investigating the incident.