SINGAPORE - A two-hour train disruption on the North-East Line (NEL) on Monday morning was caused by a new train undergoing testing, the Land Transport Authority (LTA) said.

Preliminary investigations by LTA found that a new train undergoing testing had damaged the NEL's overhead catenary system as it was being withdrawn to the depot.

The catenary system is the power supply system installed on the ceiling of the train tunnel.

Speaking to reporters on Monday morning, Land Transport Authority (LTA) chief executive Chew Men Leong said the train had pulled a wire, causing the power fault.

He said: "We will have to investigate very clearly as to why the new train caused this effect on the overhead catenary system and pulling the wire."

SBS Transit and the LTA initially activated free bus boarding along all affected stations. Free bus services were subsequently extended islandwide to help students taking O and A level examinations on Monday to get to school.

This includes all SBS Transit and SMRT services. The services ceased at 9.30am, SBS Transit tweeted.

Mr Chew said the LTA informed the Singapore Examinations and Assessment Board, and worked with the Education Ministry to make sure schools were aware that students might be arriving late.

SBS Transit chief executive Gan Juay Kiat said the operator had issued excuse slips for students.

Mr Gan added: "Once we stabilise the situation we will be investigating together with LTA on the train that caused the dewirement when it was returning to the depot."

SBS Transit deployed more than 100 shuttle buses and about 120 goodwill ambassadors to manage the disruption. The first shuttle bus started at Punggol at around 5.30am.

Southbound train services towards HarbourFront resumed at around 6.50am, while northbound services towards Punggol were back up at about 7.20am, said Mr Gan.

However, trains initially ran at slower intervals of about 6.5 minutes compared to the usual peak hour intervals of 2.5 to 3 minutes, resulting in crowded station platforms and commuters struggling to board packed trains.

Full train service resumed at 9.30am "with 20 trains operating at 3-minute headway", LTA said on Facebook. Islandwide bus services were stopped at the same time.

Rail operator SBS Transit posted a tweet at 5.23am saying that train service had ceased due to a power fault.

Meanwhile, train service on the East-West Line from Tanah Merah to Changi Airport was also briefly disrupted.

SMRT tweeted that trains were not available between the two stations at 6.23am. Service had resumed by 6.38am.