The hairline cracks on 26 China- made MRT trains were caused by impurities in the aluminium car- body material, the Land Transport Authority (LTA) said yesterday.
Laboratory tests showed that "localised impurity" occurred during the manufacturing process.
The trains are manufactured by China Southern Railway (CSR) Qingdao Sifang Locomotive and Rolling Stock Company.
The LTA was responding to media queries about an earlier report that a train had been shipped out of the Bishan MRT depot last month back to the manufacturer. In a detailed response, the LTA said that during a routine inspection in July 2013, cracks were found on the trains' car-body bolster - a structure that rests between the train carriage and its bogie. No cracks were found elsewhere.
The LTA began sending the affected trains back to the factory in China from July 2014, to replace the entire car body of each train. This was the most effective way, it said.
Shipping and replacement costs are borne by the contractor as the trains are still under warranty, the LTA added.
The LTA had awarded a $368 million contract in 2009 to CSR Sifang and Japan's Kawasaki Heavy Industries to build 22 new trains for the North-South and East-West lines. More trains were later added.
Addressing other points raised by online news portal FactWire, the LTA said there was an incident on one train where its battery cover "flew open" due to a build-up of gases during testing. The manufacturer changed its supplier and improved the battery housing design for all affected trains.
Separately, the authority said it found cracks on glass panels in five MRT trains. In May 2012, part of the glass panel next to a row of seats shattered. The cracks were caused by errors during installation, and are unrelated to the hairline cracks on the train structure, the LTA said.
It added: "It is not unusual to detect some defects on new trains."