Train services on the East-West and North-South Lines recovered yesterday morning after a major breakdown on Tuesday evening.
They were crippled because a safety device was triggered, causing power supply to trains to be cut off, said SMRT's managing director of trains Lee Ling Wee.
Commuters had to look for alternative transport options until train services resumed 3½ hours later.
Tuesday's incident is the first time train services on both lines broke down at the same time.
It also marks the worst disruption for SMRT since Dec 15, 2011, when train services were down for about six hours on the North-South Line between Bishan and Marina Bay, affecting about 127,000 commuters.
Two days later, train service on the North-South Line was disrupted between Toa Payoh and Marina Bay, affecting about 94,000 commuters.
SMRT, however, is still not "100 per cent sure" of the root cause of Tuesday's breakdown.
Mr Lee narrowed it down to three possible causes, which he shared at a joint press conference with the Land Transport Authority (LTA) yesterday.
The power cables that connect to the third rail could have been damaged, causing a power trip that triggered the safety device. The third rail is a conductor that runs alongside the entire track and provides electricity to each train.
Something faulty at the power substation, where power runs through cables to the third rail. TRAIN
Something broken dangling from the train that touched the third rail, causing the power to trip. SMRT has since ruled out this cause after checking all the trains on Tuesday night.
On the same night, three faults were also found and rectified, Mr Lee added. The faults were:
The insulation of two power cables along the North-South line was damaged.
A faulty relay system (an electrical device activated by a current or signal in one electric circuit to open or close another circuit) was found at Kranji's power substation.
There was water leakage along a section of the Tanjong Pagar tunnel, close to a third rail insulator.
Train services resumed without a glitch after these faults were rectified, but it is not clear if they had a "system-wide effect" on the rail network, said SMRT chief executive Desmond Kuek.
The faults found in SMRT's investigations were not identified during routine maintenance checks, which are conducted every six months or yearly, Mr Lee conceded.
He said that in an ageing system, some of the train parts may fail in between the interval checks.
The train operator is looking at monitoring devices that can identify faults on a real-time basis, Mr Lee added. It is also considering stepping up the frequency of checks.
LTA's chief executive Chew Men Leong added that a new voltage-limiting device has been piloted for Downtown Line 1, which can contain power trips into smaller zones.
Mr Lee said it will take some time for SMRT and LTA to look through all the data becoming coming up with a solution.
Mr Chew also promised to leave no stone unturned in this investigation.