SINGAPORE - Full train service has resumed on both the North-South and East-West lines, operator SMRT said its latest update on Facebook at 10.33am on Wednesday morning.

The North-South line, which SMRT initially said might be closed from 11am-4pm for its engineers to continue investigations into what caused the crippling breakdown on Tuesday night, will remain open.

Free bus services will also be halted at noon.

Transport Minister Lui Tuck Yew, who was at Ang Mo Kio station at around 7.15am, apologised to commuters for the "massive disruption" to their Tuesday evening routines. He added that SMRT was still working to identify the root cause of the breakdown.

Mr Lui also revealed that service had resumed at 5.30am with 100 trains - down from the usual 121 during morning peak periods.

LTA later issued a statement saying that there was no power tripping observed in the morning, with SMRT gradually adding more trains into both lines to improve train frequency.

Trains are now operating at intervals of two to three minutes.

There is speculation that a faulty train could have triggered a power surge which led to a shut down of power across the entire North-South East-West Line MRT network during the evening rush hour on Tuesday.

The disruption, which occurred after 7pm on Tuesday evening, had caused an unprecedented delay on the two lines.

It is the first time that services on both the North-South and East-West Lines were affected at the same time, in what is possibly the worst MRT breakdown Singapore has experienced. Going by the scale of the breakdown and the time it occured, it is estimated that more than 250,000 commuters may have been affected.

Service on the two crowded lines was disrupted for more than two hours, affecting thousands of commuters heading home. Many had difficulty finding alternative transport as the crowd swelled, even though free bus services were activated at the affected stations.

Service on the North-South line resumed at 10.35pm, more than three hours after the disruption which occurred after 7pm. Service on East-West line resumed at slower speed after 9.30pm.

The disruption came just days after a track fault disrupted service on the North-South MRT line that affected thousands during the morning rush.

"A faulty train could have caused the power to trip; arcing was observed on the underside of this train. The power surge caused the protective relays across the entire North South East West Lines network to be activated, leading to a shut-down of power across the two lines," the Land Transport Authority (LTA) said in a statement at 11.08pm.

"We will continue to comb through the system through the night to see if there could be other faulty trains or other causes," it said, adding that LTA engineers will be working through the night with operator SMRT to recover train services and ensure that services run properly on Wednesday.

All passengers who were on board trains during the disruption were safely detrained at station platforms. "All available bus assets were activated across the whole island to provide free bus service for affected commuters, with the police helping with crowd control," said LTA.

LTA's crisis management team, led by chief executive Chew Men Leong, was at the Land Transport Operations Centre to lead the recovery efforts, it said, adding that LTA engineers were also at the incident site to assist SMRT's recovery efforts.

LTA said it will launch a full investigation into the cause of the incident. "We apologise for the inconvenience caused to commuters."

SMRT, in a statement issued past midnight, said power trips along the two MRT lines were detected from 6pm.

"These power trips were linked to the voltage protection circuit of traction power, which is designed to safeguard the safety of commuters at station platforms. Our engineers initially managed to reset these power trips." said Mr Lee Ling Wee, managing director, SMRT Trains.

"But as we tried to identify the cause of the power fault, the frequency and impact of power trips intensified. At 1915 hours, we decided to detrain all passengers and suspend NSEWL train services for safety reasons in order to isolate the cause of the network-wide power trips," he added.

"Our preliminary assessment points to a possible power-related equipment failure at substations or along the track, or the undercarriage of a potential defective train which we are investigating tonight. But we are not ruling out other possibilities."

SMRT President and CEO Desmond Kuek said: "SMRT has mobilised all engineering and technical staff to comprehensively check the entire network and train fleet. SMRT and LTA engineers are working round the clock to conduct tests, and sections of the network are being isolated to pin point the fault.

"While it is still early to conclude the cause of the fault, we are doing our utmost to recover the system with the view to resume full train services as soon as possible. We deeply apologise for the inconvenience caused to all our commuters."

Transport Minister Lui Tuck Yew said in a Facebook post: "I am extremely concerned with today's incident. This is the first time that services on both the North-South and East-West Lines were affected at the same time. I am sorry that so many commuters experienced massive disruptions to their journeys during the evening peak hours."

In an earlier update, SMRT said it was running tests on the North-South line to systematically isolate the source of the power fault. On the East-West line, SMRT engineers were "conducting tests involving 10 trains at a time to check the entire line" after all affected trains have reached the nearest MRT station and passengers have been safely transferred to the platforms.

SMRT first tweeted about the disruption at about 7.15pm. Free bus services were made available at all stations on both lines to serve the thousands of commuters caught in the massive disruption.

But some commuters were still trying to find alternative transport to get them home two hours after the service disruption, with some bus stops and taxi stands packed with the waiting crowd.

Banker Jon Yeo, 46, was on an East-West train headed west from Tanjong Pagar when it ground to a halt at around 7.10pm. It was stuck for close to 15 minutes with partial cabin lights and air-conditioning. The lights and aircon came fully back after that, but the train was still stationary.

"It moved for a bit, and then stopped again. "They finally gave up and got us off at Redhill," he said. "All stations are affected. They actually pulled the shutters down," an agitated Mr Yeo added. Asked if he was going to wait for the free bus services, he said: "No way, I'm not going to wait anymore."

Another banker, Prashanth Shankar 32, said he was at City Hall station when the incident unfolded. "The announcement came only after 15 minutes. A long line built up for the fare refund."

Seeing the chaos and sudden crowd build-up, Mr Shankar called for his uncle to pick him up in his car. "The taxi lines were very long too," he added. "There're a lot of frustrated people."

Stockbroker B C Cheong, 48, boarded the train at Outram, but it started to act up soon. "The train blacked out, and stalled," he said. "We got off at Buona Vista, then we waited for more than half hour for the free bus but still no bus." He said: "So I got on to a normal plying bus to Clementi and I am still waiting for a bus to go back to Boon Lay." Mr Cheong added that SMRT's recovery process was "poor" .

Communications personnal Eugene Mok, 30, took the North-East line to Outram. When he got off, he walked right into the mayhem. "There was no staff at Outram," he said. "We (commuters) were like lost sheep."

He made his way to the nearest bus stop near Singapore General Hospital, and found the bus and taxi lines were "so long". "So I decided to walk to Bukit Merah side of SGH about 1.5km away to try and get a bus. I'm still waiting at 8.30pm."

SMRT first tweeted about a traction power fault at about 7.15pm, telling commuters to expect an additional 20 minutes of travelling time between Clementi and Joo Koon stations, as well as between Marina Bay and Yishun. But the fault later extended to all stations along both lines.

Large crowds formed at bus stops outside the affected stations, as commuters spilled out to find another way home.

Marketing communications manager Philip Pang, 43, waited for about an hour outside Novena MRT for a taxi. At close to 10pm, he was still waiting.

Ms Eunice Chiong, 22, said she was switching from the Circle Line to North-South Line at the Bishan MRT station. A train stopped at the Bishan MRT platform with its doors open from around 7pm to 7.20pm while commuters waited, the digital marketing executive said. Then the lights in the train went dark just as they heard an announcement that the MRT Line was down, and commuters should take other transport.

The train passengers streamed out of the train onto the platform. There was also an announcement that there will be free shuttle buses, but no directions as to where they were.

At the bus stop near Bishan MRT, a woman told her that all bus services were free, but she could not get on the buses as they were too crowded. "The buses could not even close their doors as there were too many people," Ms Chiong said.

Four buses came and went before she decided to walk to a friend's house in Ang Mo Kio where she is waiting to get a lift home to Woodlands.

Another commuter, who wants to be known only as Michelle, reached Raffles Place MRT station at about 7.15pm.

She noticed that the queue for the train on the East-West platform was "very very long". After a while, there was an announcement that there was a power fault, and commuters were requested to leave the station.

"People kept entering the station, because there was no one managing the situation on the ground," said Michelle, 28, who is in the banking industry. She estimated that there were about 300 people who were looking for alternative transport.