The April 18 Circle Line disruption that affected 18,000 passengers was caused by a short circuit in a defective power cable.
In the ensuing power trip, the backup battery supply kicked in, but the alarm that was to alert SMRT staff about the use of the backup battery did not work.
So the battery drained out after four hours and the disruption that followed inconvenienced thousands.
In a press conference on Tuesday, Mr Tan Ek Kia, SMRT executive director and interim chief executive officer, said investigations are ongoing to determine why the alarm system failed.
Besides the Circle Line incident, there have been three others on the East-West Line since April 8.
On Sunday, there was also a widely publicised incident of a Bukit Panjang LRT train stalling, which required passengers to detrain and walk along the track to the station.
In addition to explaining the technical details of how these disruptions happened, Mr Tan apologised for them.
He said: "We find this not acceptable. We have been working very hard over the last few months to try and improve our performance."
Because of the disruptions, SMRT has decided on a plan for additional maintenance, upgrading and renewal of the train system.
It is expected to cost about $900 million and will include both ongoing measures (such as re-signalling of the North-South and East-West Lines) and new projects.
The plan starts this year and will extend to 2019.
Major renewal project
One of the major renewal projects is the change-out of wooden sleepers along the ballasted tracks on the older lines.
As a comparison, SMRT usually spends about $30 million each year on the hardware component of maintenance and repair (excluding some costs such as manpower cost).
Mr Tan said SMRT will work more with the Land Transport Authority to ensure that the actions in the plan will be "robust" in achieving the objective of a safe and reliable train operation.
He also acknowledged that there has been an upward trend in the number of train disruptions due to faults.
From about 1.5 withdrawals per 100,000km in January 2009, it was about 3.5 last month.
While SMRT is trying to arrest the trend, Mr Tan said it is important to acknowledge the backdrop of an ageing train system and increased usage.
For example, 66 of the SMRT-run trains - about half of a total fleet of 128 - are 25 years old.
This article was first published in The New Paper.