Of the 26 defective MRT trains, five have had their aluminium car-body shells replaced after being sent back to China for repairs. A sixth was sent back recently and starting from next year, two will be sent back each time instead of one, to speed up rectification works.

A routine inspection in July 2013 had uncovered hairline cracks on the surface of the car-body bolster - an aluminium alloy structure beneath the train carriage.

The Land Transport Authority (LTA) yesterday said the replacements - which can take up to four months for each six-carriage train - are under way for the sixth train.

"To minimise the impact on our train operations in Singapore... only one train is sent back to the factory in China at any one time," it said.

In explaining why the 26 trains, which are still under warranty, were sent back to Chinese manufacturer CSR Sifang for repair works, the LTA also cited the "lack of facilities and space for repair works of this nature" at MRT depots here.

After the cracks were found, LTA engineers, CSR Sifang and an independent third-party assessor TUV Rheinland had assessed the 26 trains and concluded they were safe to operate, the LTA said.

"We did not send all of the trains back at once as they were still fit and safe for service, and we wanted to ensure sufficient train availability for commuters," it added.

Starting next year, there will be more new trains on the North-South and East-West Lines (NSEWL) and trains undergoing upgrading will be ready, so two trains can be sent for replacement works at the same time, the LTA said. This will allow the replacements to be completed by 2019, instead of the initial 2023 date.

SIM University Adjunct Associate Professor Park Byung Joon said the first responsibility of testing trains should be with the manufacturer.

"It's like buying a car. When there is some trouble, the manufacturer will fix it," he said. "Because they didn't announce it earlier and the story was leaked to the Hong Kong press first, it sounds a lot worse than it actually is."

SMRT spokesman Patrick Nathan said the affected trains are the newest in the fleet serving the NSEWL, and the defects were not safety-critical. "Any train that is assessed to be defective or unfit for passenger service is not put into service," he said. "The decision to send the trains back... will not affect train service availability or frequency, as this is being done in small batches."