UPDATED (1226am 11 January 2013, adds Minister's comments)
58,000 commuters were affected during the nearly seven-hour North East Line (NEL) train disruption on Thursday.
Service between HarbourFront and Dhoby Ghaut stations was down from 10am to about 430pm when full service resumed.
In a statement, the transport operator said the fault occurred at 9:51am and that the fault had been traced to three broken U-turn bolts which resulted in a power trip in a section of the tunnel between Outram Park and HarbourFront.
Last August, a similar fault occurred at the same stretch. SBS Transit has decided to replace all 1,900 U-bolts in the tunnel following Thursday's incident.
Meanwhile, Transport Minister Lui Tuck Yew said a joint task force from the Land Transport Authority (LTA) and SBS Transit has been formed to tackle the troubled stretch between Outram Park and Harbour Front.
"Today's fault took place very near to the HarbourFront area; this is the same stretch that experienced some problems twice over the last year between Outram and HarbourFront," said the Minister as quoted on TodayOnline.
"So I have asked LTA to work with SBS Transit to get to the bottom of this. They were already in their first meeting for their joint team and this was one of their focus areas, so they know there's a problem, we have to go down to the root of it and try and resolve it to best as we can."
Affected commuters were given free rides on bus services at affected stations during the disruption.
SBS Transit said it was "very sorry" for the latest disruption which inconvenienced commuters.