New trains unloaded for the Tuen Ma Line, an integral part of the Sha Tin to Central Link

MTR has delayed the launch of its new signalling system planned for rollout today, after finding a bug that could send trains off their planned routes and to unintended destinations.

MTR officials prepare passengers for a shortening of East Rail Line trains – the plan is now delayed by a signalling system bug

The new system was planned for the East Rail Line’s shift to shorter 9-car trains and is considered a milestone in the Sha Tin to Central Link project.

On 28 August, MTR said the signalling system had “completed all necessary testing and drills and obtained approval from the relevant Government departments.”

But final testing over the last few days revealed an unexpected fault “which may lead to a possible scenario of a train travelling to a destination that is different from its planned route,” says the company.

The risk of such an incident happening is low, says MTR, while other fail-safe measures in the system would prevent any train collision in the event of such a signalling failure.

MTR originally set to developing a workaround. the company says, but in “final review” decided to work out the kinks in the signalling system instead.

“The replacement of the signalling system is a complex project. We will ensure operations of the new system are smooth and sound, to provide safe and reliable service to passengers,” says Sammy Wong, MTR’s operations chief.

The existing East Rail Line signalling system suffered a major failure on 11 January 2018, caused by a hidden software coding error in the Train Control System. That bug only surfaced, according to MTR at the time, as traffic demand had increased. While all safety systems continued to work during the incident, the failure caused mass disruption for tens of thousands of passengers for several hours.