Rail operator SMRT will conduct a thorough inspection of rail joints on the North-South and East-West lines where cracks could potentially occur.

In a statement on Monday, SMRT said preliminary analysis found three similarities between the rail crack incidents on April 29 and May 18 .

The rail cracks occurred at rail joints welded between old and new rails; they also happened on concrete tracks located near the tail wall of the underground stations; and were welded by the same contractor in June last year and February this year. The operator will focus its inspections on rail joints with these characteristics.

SMRT vice-president for corporate marketing and communications Ms Kalai Natarajan said a full investigation into four main areas is underway to determine the root cause of the incidents. The operator will study the material and workmanship of the rail joints, as well as examine possible stress factors and whether the welding process was carried out properly.

Ms Natarajan said: "SMRT had responded to both the incidents this year in compliance to safety and recovery procedures, and train services would not have been allowed to continue if it was assessed that passenger safety may be, in any way, compromised."