How does the Circle Line train signalling system work?

Reader Andrew Kurniawan Ng wanted to know how the MRT signalling system works, and why a glitch in it can cause such "havoc" as seen on the Circle Line last week. Senior Transport Correspondent Christopher Tan replies:

Signalling refers to a broad range of systems which allow trains to run safely.

In the Circle Line, the trains are fitted with a communications-based train control "moving block" signalling system.

Using telecommunication signals to "talk", the trains are able to keep a safe distance from one another, irrespective of speed.

Compared with a "fixed block" system, a "moving block" system is more sophisticated and allows trains to run closer to one another. The North East Line - which is driverless like the Circle Line - runs on a similar system.

The North-South and East-West lines are also converting to this system.

In addition, the Circle Line has an automatic train protection system to govern train speed, an automatic train supervision system to track and schedule trains, and another system that prevents incorrect signals from being set.

Because of an unknown signal interference in the tunnels, communication between the trains and the track was interrupted. This had the effect of a train going blind and deaf momentarily, and caused yet another fail-safe system to kick in - the emergency brakes.

Together, they form many pairs of "eyes" and "ears" of the network. It is unusual for a system with such a high redundancy to trip up.

Yet, that was exactly what happened from Monday to Friday last week.

Because of an unknown signal interference in the tunnels, communication between the trains and the track was interrupted. This had the effect of a train going blind and deaf momentarily, and caused yet another fail-safe system to kick in - the emergency brakes.

Because the trains did not "know" where they were going, they stopped. Each time a train stops, its signalling system has to be reset to get it to move again. These events, repeated many times a day, caused journeys to be longer and more uncomfortable.

During the episode, operator SMRT decided to man the driverless trains. The drivers took over when the trains went "blind" and "deaf". The staff on board, however, did not drive the trains on manual mode completely - not for extended periods of time anyway. So, commuters continued to face a degraded service.

But, as mysteriously and suddenly as the interfering signal appeared, it has since disappeared. The Circle Line has experienced no loss of signalling communication since last Friday evening.

SMRT, the Land Transport Authority and systems suppliers Alstom are still investigating the source and nature of the interference, so that they can prevent a recurrence of last week's events, which left thousands of commuters peeved.

A version of this article appeared in the print edition of The Straits Times on September 10, 2016, with the headline 'How does the Circle Line train signalling system work?'. Print Edition | Subscribe