SINGAPORE - Transport Minister Lui Tuck Yew has apologised to commuters affected by Tuesday (July 7) evening's "massive" train disruption on the North-South and East-West lines.

Speaking to reporters on Wednesday morning at Ang Mo Kio Station, where he was present since 7.15am to observe the running of trains, Mr Lui said: "I'm sorry that it affected so many commuters during the evening peak hours.

"Many of them faced uncertainties and disruptions to their normal routines."

But Mr Lui stopped short of calling for a Committee of Inquiry (COI) into the incident, saying it was more useful for SMRT and the Land Transport Authority (LTA) to concentrate its efforts finding the "definitive cause" of what went wrong.

"If we can find it (the definitive cause), we can take the necessary corrective actions and let commuters have a peace of mind knowing we have resolved the faults that caused the disruption," he said.

The last time a COI took place was in December 2011, when Prime Minister Lee Hsien Loong ordered an inquiry after three breakdowns in four days on the North-South line.

The third breakdown, on Dec 17, affected 12 stations from Marina Bay to Ang Mo Kio and lasted seven hours.

Tuesday night's disruption is believed to be the worst MRT breakdown Singapore has experienced, with an estimated 250,000 commuters affected.

It is also the first time both the North-South and East-West lines were affected.

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

Mr Lui said SMRT was still trying to identify the root cause of the disruptions and was conducting visual and impedance checks on all its trains.

"There was a train that experienced arcing (electrical spark), we will have to drill down further to see if this is the only cause or if it contributed to the incident," he added.

"During checks on Tuesday night SMRT also found a couple of wires that they had to change. This was around the Bishan area but we do not know if it was the cause."

The North-South line will be closed from 11am to 4pm later today for SMRT and LTA engineers to continue their investigations.

Mr Lui also revealed that about 100 trains are currently in operation, down from the usual 121 during morning peak hours.