SINGAPORE - They say no news is good news, and that certainly seems to apply to the Circle Line.

The line has been operating normally since Friday evening following a string of delays last week.

Since last Monday morning, service on the five-year-old MRT line had been glitchy because a mysterious interference in the tunnels was creating havoc with the trains' signalling system.

A signalling system ensures that trains operate safely.

But as of Friday 4pm (Sept 2), the interfering pulses have ceased, and trains have been operating normally since, said the Land Transport Authority (LTA) and operator SMRT Corp on Monday (Sept 5).

SMRT's engineers have suspected a communications interference from the start of the anomaly, but were not been able to ascertain it.

Together with experts from both LTA and systems supplier Alstom, they nailed it down to an interference from an unknown source late last week.

"SMRT and LTA are continuing with the investigations," read a joint statement from LTA and SMRT. "Should and when an incident recur, SMRT and LTA have worked with the Infocomm Development Authority and mobile network operators to suspend telecommunication signals along stretches of the Circle Line for short periods of time for further tests.

"If temporary telecommunication signal suspensions have to be carried out, there will be in-train and station announcements along the affected stretches."

Despite the unprecedented service degradation last week, the Circle Line has in fact been a reliable line. According to LTA statistics, the line had a breakdown for every 177,000 train-km clocked in the first quarter. Next in line was the brand new Downtown Line, with 171,000 train-km between breakdowns.

The orbital line however, was not much better than the older lines when it came to major incidents.

Since 2011, it had 11 breakdowns exceeding half an hour each. This compared with 12 for the 13-year-old North-East Line, and 26 for the North-South and East-West Lines (the oldest stretch being 29 years old) combined.