Commuters were hit with a second train disruption this week, the 12th such incident lasting more than 30 minutes this year.

More than 70,000 commuters were affected when train services between the Yishun and Bishan MRT stations in both directions were disrupted for nearly two hours from 5.45am yesterday due to a traction power fault.

Preliminary investigations indicate that the fault was caused by a damaged power breaker at Ang Mo Kio Station on the North-South Line and that this was unrelated to Monday's 20-minute delay between Yishun and Marine South Pier, which was caused by a faulty train.

It is understood that the breaker was damaged after a flashover, or an unintended discharge of current. This in turn caused a fire, which was put out by the Singapore Civil Defence Force at 5.50am. No injuries were reported.

SMRT managing director Lee Ling Wee stressed that the disruption was caused by a localised fault at Ang Mo Kio, and the root cause of the issue was "definitely not related" to the problem that triggered the massive train breakdown in July as "the symptoms are very different".

On July 7, both the North-South and East-West MRT lines were crippled for three hours due to a contaminated insulator which caused a higher than normal voltage difference between the running rail and the ground, leading to power trips.

Yesterday, SMRT activated 50 additional buses which were free for commuters and operated bus bridging between Yishun and Bishan.

Commuters at the Yishun station were advised by staff to head towards Jurong East to take a long detour into the city, while those at Bishan were asked to travel south.

SMRT managing director Lee said more staff were sent to the Jurong East interchange to manage the crowds.

Staff members gave out excuse slips explaining the delay and tickets for complimentary train rides to commuters.

SBS Transit was also called in to provide additional buses on their regular routes in the affected areas.

"In terms of recovery, I would say that we tried our very best and moved as quickly as possible," said Land Transport Authority (LTA) chief executive Chew Men Leong at a doorstop yesterday.

He added that most commuters were able to get to their destinations via alternative services provided, although they experienced delays.

The delays meant that some commuters took up to two hours to get to work.

Some students who were headed for the GCE A-level literature examination, which started at 8am, were also late.

Snaking queues formed at Yishun Bus Interchange for buses headed towards Bishan as commuters scrambled for alternative routes to their destinations.

Long queues were also seen at taxi stands at Bishan and Yishun and people spilled out of the bus stops opposite Yishun MRT Station onto the roads while attempting to hail taxis or board buses.

Some, like Ms Grace Angie, a 33-year-old who works in human resources, tried calling for a taxi or private car.

"But the roads are too congested and none of the cars got to me. I waited more than 20 minutes then took a bus instead," she said.

Full train service resumed at 8.10am, with trains running at slower intervals of two to four minutes. By 9.40am, the situation was almost back to normal, with trains running at intervals of between one and two minutes.

"I deeply apologise for the inconvenience caused to commuters," said Mr Lee.

Mr Chew said that the LTA and the operators must continue to enhance rail reliability and would continue investigating the root causes of the incident.