The answer to why passengers were stranded on a broken-down train for an hour two weeks ago may lie in automatic couplers which join one train to another, according to preliminary findings of an investigation by SBS Transit.
The North East Line operator said a rescue train sent to push another train which had been stalled by a short circuit could not do so because the couplers failed to work.
It is believed to have been the first failure of its kind and it led to 250 passengers being stranded for about an hour on June 19.
For the coupling to work, one train merely has to approach another at a low speed to come into contact with it.
"The couplers of both trains will automatically connect the two trains together mechanically," said SBS Transit spokesman Tammy Tan.
"Electrical control signals and compressed air can also be connected between the two trains via the coupler," she added.
After a second attempt at coupling the trains failed, the operator decided to evacuate passengers via an emergency exit ramp.
The line was then closed. Mysteriously, recoupling worked after this and the stalled train was pushed back to the depot.
"Why the coupling failed is the exact focus of our investigations," Ms Tan said.
The Land Transport Authority (LTA) is also investigating the incident, which affected 50,000 commuters over a two-hour period.
The incident came just two weeks after another electrical fault - caused by faulty insulation along the line's power supply - disrupted service on the 10-year-old line for an hour.
SBS Transit was criticised over its handling of the second breakdown.
Stranded passengers complained that communication was poor while transport experts said the operator had not done enough to mitigate the incident.
Observers said SBS Transit fell short despite clear guidelines on evacuating a stalled train.
A high-level Committee of Inquiry into two massive disruptions in 2011 determined that passengers must be "detrained" within 30 minutes of a breakdown.
Transport Minister Lui Tuck Yew said he was "dismayed" at how passengers were stuck on the train "for an unacceptably long period".
The LTA said its investigations will extend beyond the mechanical causes of the breakdown.
"We will also be looking into other areas like incident management," a spokesman said.
She said the probe will take another three months to complete.