Downtown Line operator SBS Transit yesterday gave more details of how a child passenger could have activated a detrainment switch that triggered a power cut which disrupted the newly opened line on Friday.
A spokesman said preliminary findings showed that "the internal laminate of the metal cover which holds the detrainment door switch had debonded".
This, she said, caused the cover to be loosened.
"When the cover was moved, the detrainment switch was triggered," she said. As a safety precaution, power is cut off when the switch - which lowers a ramp for emergency evacuation - is activated.
SBS Transit said closed-circuit television footage showed a little girl may have caused this. It declined to share the footage.
Friday's incident disrupted the Downtown Line service for more than an hour from about 9pm. It was the second breakdown in six days, after a train fault disrupted service for 15 minutes last Sunday - its first day of operation.
Bombardier, the manufacturer of the trains used on the Downtown Line, said the scenario painted by SBS Transit was unlikely to occur when the train was moving but said it would need to look at the video footage to understand what happened.
Bombardier senior engineering manager Paramjit Singh told The Sunday Times that lifting the detrainment switch cover would cut power only if the train was stationary between stations.
"And only when power is cut can you move the detrainment switch," he said.
Netizens have questioned the cause of the breakdown. Commenting on Facebook, Charlton Gan asked: "Why is the device so easily accessed by the kid in the first place?"
Another, Neo Eng Hin, asked how the system could be "so fragile and sensitive".
SBS Transit said it has since "enhanced the cover for a tighter fit" on all its Downtown Line trains. It has also stationed staff at both ends of trains to prevent a repeat of the incident.
Its senior vice-president for corporate communications Tammy Tan said the company was working with the Land Transport Authority (LTA) to review the design of the detrainment device.
In a separate statement, the LTA, which oversaw the construction of the line and the procurement of its trains, and which also regulates train operators, said it would take a closer look at the incident and identify appropriate measures to prevent a recurrence.