SMRT has imposed a speed restriction of 40km per hour on the stretch of the North-South line affected by the rail crack.
The rail operator said the speed limit will be enforced for the entire day and repairs to rectify fault will be conducted after operations on Thursday night.
Southbound MRT services between Woodlands and Ang Mo Kio were delayed from 6.25am Thursday morning by a rail crack - the third in recent weeks.
An SMRT spokesman said earlier that the crack was detected between Yio Chu Kang and Ang Mo Kio stations, and that G-clamps have been used - as previously - to temporarily stabilise the affected area.
Travel time was delayed by as much as 40 minutes between Woodlands and Ang Mo Kio, but has now been reduced to 15 minutes.
Rail cracks have been rare in the Singapore network till recently. Before the last three incidents, there was an average of only one crack per year.