SINGAPORE - Experts say the four-day-long glitch causing delays on the Circle Line is a "serious" issue, even as the operator and the authorities say they are working "round the clock" to resolve the fault.

Since Monday morning, trains on the five-year-old line have had to apply emergency braking several times a day as they intermittently lose signalling communication with the tracks, leading to longer journeys.

National University of Singapore transport researcher Lee Der-Horng said: "It sounds quite serious to me... it's been four days."

Prof Lee said the line operates on a wireless communications-based system which "tells trains how fast they should be going, when they should slow down, and when to stop".

He added that such a system has to be "reliable" and that "there should not be any blindspot".

SIM University senior lecturer Park Byung Joon even warned that the intermittent loss of communication could "proliferate and lead to a total shutdown".

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Operator SMRT has now put drivers on board the driverless system, but Dr Park said doing this in the long run "is not feasible".

"The system is not designed for that," he said. "That's a short-term solution."

Dr Park noted however, that the emergency braking system will ensure safety when the signalling system fails.

"It's not a high probability that it (the braking system) will fail as well," he said, but added that when a technical fault persists for days, the authorities have to manage public perception about safety and reliability.

Dr Park added that it would be useful to find out if the stoppages have increased or decreased in frequency and severity. But there was no information available on this.

In a joint statement with SMRT on Thursday (Sept 1), the Land Transport Authority said: "We are working round the clock to restore service levels."

They are also working with systems supplier Alstom to resolve the issue, which has been traced to unknown communication interference in the tunnels.

"Commuters... are advised to factor in extra travel time to get to appointments on time," the LTA said.

Commuter Andrew Ng, 28, said that he was anticipating delays and left home for work earlier on Wednesday morning, but was still 15 minutes late.

"It was really bad. It took me 45 minutes to travel from Serangoon to Holland Village," the communications specialist said, adding that the journey typically took less than 20 minutes.

Prof Lee of NUS, a regular user of the Circle Line, said he now uses the bus to complete his journey to work "because I don't want to risk being late".

Other commuters took to social media to make light of the unknown communication interference in the tunnels on Thursday.

"Must be Godzilla," Eisen tweeted.

"Perhaps there is a kaiju in the tunnels," TunasKelapa rejoined, referring to the Japanese anime term for "strange beast".

Sitoh Yih Pin, chairman of the Government Parliamentary Committee for Transport and MP for Potong Pasir, said even as investigations into the glitch continues, SMRT should deploy more staff at stations to help commuters: "This is important as the problem has now been protracted for four days."

SMRT said it is doing just that, adding "we apologise for the inconvenience and seek all commuters' understanding as we work... to resolve the issue".