There has been another rail crack between Tiong Bahru and Outram MRT stations, with trains slowing to a crawl late on Friday night.
Train operator SMRT tweeted at about 11.20pm that commuters should expect an estimated 30 minutes of additional travel time from Jurong East to Outram Park towards Pasir Ris.
Engineers have been deployed on site to investigate, it said on its Facebook page, and have confirmed that the rail crack is not at a welded portion of the rail.
The company had, earlier in the day, explained the cause of the most recent rail crack incident. It said it was corrosion that caused a rail crack on the North-South Line on Thursday, which delayed train services for up to 40 minutes.
This was unlike two other recent incidents on the same line, where rail cracks involved welded joints.
SMRT said it has begun talks with the Land Transport Authority on a complete overhaul of what it called an "ageing" rail network.
"Engineers do not preclude similar incidents from happening in future," said SMRT's vice-president for corporate marketing and communications Kalai Natarajan.
Before these four incidents, which happened in the space of five weeks, there was an average of only one rail crack per year.