Track maintenance works which could not be completed before train operation hours caused delays on the North-South MRT Line for four hours yesterday morning.

It was the second such incident in a week on the heavily used line, which had experienced a relatively trouble-free January after a host of problems last year.

Just before 6am yesterday, SMRT announced through Twitter and Facebook that city-bound trains were travelling slower because of "maintenance work", and that journeys would take 20 minutes longer. Normal service resumed at 10am.

Ongoing track improvement work had also caused delays, of between five and 10 minutes, on Feb 7.

On yesterday's delays, SMRT Trains chief maintenance officer Chia Chun Wah said the operator had to cast fresh concrete on a track bed which supports the running rails near Raffles Place, to replace the worn parts of a concrete track slab.

Unlike surface tracks, which rest on individual sleepers, underground tracks sit on sleepers which are embedded in concrete slabs.

When these are worn or damaged, they have to be hacked away and recast.

Mr Chia said the fresh concrete requires "several hours" to fully cure.

To facilitate this curing process, SMRT put in place additional support structures and imposed a further speed restriction over a 300m stretch, with trains moving at 5kmh to 10kmh. Normal speeds are between 60kmh and 80kmh.

The speed restriction caused the trains to bunch up along the line during peak periods. Its impact is far less apparent during off-peak hours when fewer trains run.

The Straits Times understands that similar work on at least one more location will be completed by next week.

Since December, operating hours on stretches of the North-South and East-West lines have been shortened on Fridays and weekends to allow the operator more time to fix problems.