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Mystery signal disruption: Train systems to be enhanced

SINGAPORE — Some two weeks after it began investigations, the Land Transport Authority (LTA) and SMRT have not been able to determine the source that caused the “intermittent loss of signalling communications” on the Circle Line (CCL) late last month, which led to several days of train service delays.

Crowd at the Circle Line at Bishan MRT Station on Sept 1. TODAY file photo

Crowd at the Circle Line at Bishan MRT Station on Sept 1. TODAY file photo

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SINGAPORE — Some two weeks after it began investigations, the Land Transport Authority (LTA) and SMRT have not been able to determine the source that caused the “intermittent loss of signalling communications” on the Circle Line (CCL) late last month, which led to several days of train service delays.

But they will be enhancing the resilience of the line’s signalling system, such as by installing spectrum analysers — which measure the strength of signals at a particular frequency — within the CCL tunnels to detect the presence of any interfering signal. This measure will be extended to the North-East Line, which has the same signalling system. 

The LTA and SMRT will also conduct feasibility studies on enhancements such as electromagnetic shields, which can be installed on the trains to minimise the impact of interfering signals.

“We will also explore the possibility of changing the signal system frequency and/or modifying the system to provide redundancy in the event of signal interference,” they said in a statement on Wednesday (Sept 21).

The signalling systems on newer rail lines, such as the Downtown Line, and current lines undergoing upgrading, such as the North-South and East-West Lines, are already equipped with new features that provide redundancy in the event of signal interference, they said.

On Aug 29, persistent signal interference, which automatically activated the trains’ emergency brakes, had caused travel delays on the CCL. This went on for about five days. In an unprecedented move, the LTA and SMRT suspended mobile signals at four stations as part of investigations.

Following a series of tests, the LTA, SMRT and train manufacturer Alstom assessed that an interfering signal could have disrupted the trains’ signalling systems, and led to the loss of communications.

“Over the past two weeks, engineers have tried to establish the source of the interfering signals. However, as the incidents had ceased by the afternoon of Sept 2, 2016, they were unable to determine the source,” the LTA and SMRT said, adding that they will continue to closely monitor the performance of the CCL.

Describing the measures as “a good start”, Singapore Institute of Technology railway signalling control and communications academic Andrew Ng said that any device capable of wireless communication could be a source of the interfering signal, from telematics equipment to mobile radios.

Apart from a radio frequency interference, the issues that plagued the CCL could stem from electromagnetic interference. Spectrum analysers would help determine if there are interfering signals and their frequencies, but more investigations would still be needed to identify the source, said Assistant Professor Ng.

Electromagnetic shields can minimise interference with the railway signalling system, but will not eliminate the problem fully. “There’s always a certain degree of tolerance (and) it’s not a 100 per cent thing that you can cover up,” he said.

Likewise, changing the signal system’s frequency to a “non-disturbed” one would help, but “we do not know when there would be another interfering (signal) that (could) disturb (that) operating frequency”, he said.

What is crucial, Asst Prof Ng stressed, would be to identify and eliminate the source of the problem.

Providing for redundancy would help by serving as a back-up communication network. “If the signal is interfered (with) again ... then we can leverage the redundancy (to) prevent a communication loss between the trains and tracks,” he added. ADDITIONAL REPORTING BY KENNETH CHENG

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