An independent consultant will be brought in to examine the rail network, even as a second day passed with engineers unable to pin down the root cause of Tuesday's massive disruption.
Staff from SMRT Corp and the Land Transport Authority (LTA) had carried out a second round of checks on Wednesday night.
In a statement yesterday, the LTA said it will engage an independent consultant specialising in transit power systems as early as this month.
The experts will have to look into the "overall conditional assessment of the entire power supply infrastructure and other railway trackside installations".
Meanwhile, LTA and SMRT will continue checking the system. The operator had earlier identified three possible causes - worn cable insulation, tunnel water leakage, and a glitch in a power substation.
But it was "not 100 per cent sure" that these led to the breakdown, and is continuing to check if there were other causes.
During the second round of checks, LTA and SMRT engineers inspected all the trains visually and tested insulation between the cables and third rail.
Data loggers monitoring the running rail voltage were also installed at specific points on the North-South and East-West lines, which experienced power trips before train services on both lines were disrupted on an unprecedented scale during the evening peak period on Tuesday.
An estimated 250,000 commuters were affected that night, as two of Singapore's oldest and most heavily used lines were crippled for more than two hours.
Train services resumed later that night, but moved slower as a precaution. Over the next two days, they continued to operate on schedule at normal speeds.
Transport experts welcomed the move to engage an independent consultant, given that Tuesday was the first time both lines were entirely shut down.
National University of Singapore transport researcher Lee Der Horng told The Straits Times: "The engineers have narrowed it down to three possible causes, so that is a good sign. But even then, they could not be sure and my fear is that the root cause is none of the three, which would be quite problematic."
Prof Lee and former UniSIM transport expert Park Byung Joon said it was important to understand how the flaws ended up affecting the whole network.
Said Dr Park: "The root cause could be something more fundamental, right down to the design and construction of the two lines.
"They may even need to look at the original blueprint. It would be good to have more manpower and a higher level of engineering expertise to find the root cause."