Train services on the North-South Line were back to normal during Tuesday's morning rush hour, after a crack was found on the northbound track between Somerset and Orchard stations on Monday.

Checks by The Straits Times found northbound trains running at one to to three minute intervals, and commuter traffic at both stations was smooth.

It was also business as usual at the Dhoby Ghaut, City Hall, Raffles Place and Marina Bay stations.

The crack was detected just after 7.30pm on Monday when many workers were heading home.

Journeys were delayed and crowds swelled at several stations along the line.

SMRT posted on its Twitter page that the fault was "believed to be caused by a rail crack".

It added that trains were "safe for service but travelling at slower speed as a safety precaution".

SMRT also said in a post at midnight that its engineers "can only rectify the fault after passenger service hours".

Cracks on tracks are rare but they can have dire consequences.

On Oct 17, 2000, a British high-speed intercity train from Leeds to London derailed because of numerous cracks on a rail. Four people died and 30 were injured.

Rail authorities and engineering experts attribute the main cause of cracks to metal fatigue. Factors such as extreme temperatures, age and uneven wheel profiles can lead to fatigue.

Monday evening's incident comes three weeks after a foreign object caused a short circuit on the North-South Line during morning rush.