Two recent occurrences of rail cracks that led to delays for commuters shared three similarities, SMRT has found.
The operator has been investigating the problems in an effort to prevent any similar delays and its preliminary analysis has discovered that both cracks occurred at joints located on a curved track and welded between old and new rails.
Both incidents - on April 29 and last Saturday - happened on concrete tracks near the tail wall of the underground stations, where trains brake to enter.
Last month a crack was found on the northbound track near Orchard station from Somerset.
The other one last Saturday occurred between Toa Payoh and Braddell.
SMRT also found that the affected rail joints had been welded by the same contractor - Gates PCM Construction.
In a statement yesterday, SMRT vice-president for corporate marketing and communications Kalai Natarajan said the North-South and East-West lines will be inspected thoroughly to detect potential rail cracks.
SMRT will focus its checks on rail joints that share the same characteristics as the two that cracked.
The Straits Times understands there are about 200 of these suspected joints across both lines that it plans to inspect by the end of this week.
Ms Natarajan said SMRT is studying four main areas in its investigation into the cracks: material, workmanship, stress factors and welding process.
The operator will send materials used to weld the two cracked rails for laboratory tests to determine their condition.
It is studying possible stress factors that may have contributed to the cracks, such as a height difference formed between the welded rails.
Trains braking at the tail wall tracks of underground stations may have also caused additional stress to the joints, she said.
SMRT will identify, inspect and test all rail joints in the system welded by Gates PCM to determine their integrity.
The welding process will also be scrutinised to determine if it had led to poorly cast welds that are susceptible to cracks.
No timeframe was given for the investigation into the cracks.
Gates PCM did not respond to queries for comment when contacted yesterday.
Ms Natarajan stressed that SMRT had complied with its safety and recovery procedures. "Train services would not have been allowed to continue if it was assessed that passenger safety may be in any way compromised," she said.