HONG KONG - The manufacturer of the software behind the city's disrupted rail services apologised on Monday evening (March 18).

In a statement, Thales said it was "deeply sorry" that a driver had been hurt, adding that it would support the ongoing investigation into the collision of the two trains.

"We fully remain at the disposal of (MTR Corporation) and the authorities to bring the appropriate assistance and information," it said.

The statement came after two trains collided in the wee hours of Monday during a test of the new signalling system that rail operator MTR Corporation intends to use.

MTR Corp had intended to use the new system alongside the old one in the second half of the year, before replacing the old one altogether.

Owing to a software glitch, the rail operator said, the signalling system failed and two trains sideswiped at around 3am.

Even though they were not moving fast, three carriages were damaged.

At the time, the network was closed to the public.

The collision occurred on the Tsuen Wan line when a train heading to Central station derailed and hit the third and fourth compartments of another train going in the opposite direction towards Tsuen Wan.

Two drivers were taken to hospital with one injured in the leg and the other reportedly suffering from smoke inhalation.

MTR's managing director Jacob Kam Chak-pui told the media that the new signalling system had allowed two trains to pass through the same crossing at the same time, a scenario he described as "unacceptable" for safety reasons.

He said that at the time of the accident the rail operator was testing the second backup computer for the signalling system. The current system has only one backup.

Thales had identified a similar problem in a computer simulation in its Toronto laboratory, added Mr Kam, who said: "It confirms that their software was problematic."

Thales is expected to send four or five experts to Hong Kong soon.

The trains had reportedly crashed while running the SelTrac signalling system developed by French company Thales.

Also known as communications-based train control (CBTC), SelTrac had been installed on Singapore's East West MRT line. In November 2017, it was blamed for a collision between two trains on the line's Joo Koon station, injuring 38 commuters.

Singapore's Land Transport Authority said a "software logic issue" prevented communication between equipment on board the train and trackside. LTA also said Thales accepted full responsibility for the accident.

Following Monday's incident, MTR said it will immediately suspend trials of the new system until the cause of the derailment was clear.

Photos of the incident showed damaged train cabins with doors unhinged. Shattered glass was strewn across the cabin floor.

MTR's operations director Adi Lau apologised for the inconvenience caused and told a media briefing that the incident was the first of its kind since the line opened almost 40 years ago. Mr Lau assured the public that the new signalling system would be implemented only after it passed all tests.