In an effort to strengthen its signalling network, the Land Transport Authority (LTA) is looking at whether installing an electromagnetic shield on MRT trains could do the trick.

Such solutions are being considered after a mysterious signal interference disrupted service on the Circle Line three weeks ago.

Tests by the LTA, SMRT Corp and systems supplier Alstom of France have indicated that an interfering signal disrupted the trains' own signalling systems and led to loss of communications. Their engineers tried to get to the root of the interference, but failed to establish it.

"As the incidents had ceased by the afternoon of Sept 2, they were unable to determine the source," the LTA and SMRT said in a statement yesterday.

The hunt for solutions could involve blocking interference with electromagnetic shields, which are used in secured facilities and high-tech laboratories.

The LTA said it will soon start feasibility studies on ways "to strengthen the existing signalling communications network". It will also explore the possibility of changing the signal frequency of the network. Separately, it will look at modifying the system "to provide redundancy in the event of signal interference".

The LTA noted that the signalling systems on newer 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". The five-year-old Circle Line and the 13-year-old North East Line do not have these features.

In the interim, spectrum analysers - which can detect stray transmissions - will be installed in the Circle Line tunnels. "This measure will be extended to the North East Line, which has the same signalling system," the joint statement read.

The Circle Line was hit by a signal interference between Aug 29 and Sept 2. The undetermined source apparently interfered with communication between trains and the track, causing trains to apply emergency braking intermittently.

This led to longer and jerkier journeys for a week. The problem stopped as suddenly as it started.