SINGAPORE - The causes and conditions leading to the train collision at Joo Koon in late 2017 are completely different from those that led to Monday's collision between two Hong Kong MTR trains, said the Land Transport Authority (LTA).

LTA deputy chief executive Chua Chong Kheng drew the distinction between both incidents in a forum letter to The Straits Times on Tuesday (March 19), which came in response to an article published in this newspaper earlier in the day.

Mr Chua also said the LTA is concerned about the collision between two trains on a trial run on the Tsuen Wan line, and is closely monitoring MTR Corp's investigations.

A train heading to Central station derailed at around 3am on Monday, when the line was closed to the public, and hit the third and fourth compartments of another train going in the opposite direction.

The South China Morning Post (SCMP) reported that the trains crashed while running the SelTrac signalling system developed by French company Thales.

This same communications-based train control (CBTC) system by Thales, which allows trains to run at shorter intervals, has been installed on the North-South and East-West MRT lines in Singapore. Both MRT lines have been operating entirely on the new CBTC signalling system since May last year.

Initial reports from Hong Kong indicate that the issue is linked to a software problem with the Tsuen Wan line's new signalling system during the testing phase, Mr Chua noted.

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The Joo Koon collision was due to "interface issues" between the old and new signalling systems on the North-South and East-West lines, he said.

"Immediately after the incident, LTA and SMRT delinked operations between the two systems to decisively remove this risk," Mr Chua added.

He said the LTA also invited Thales to set up a CBTC simulation laboratory in Singapore.

This allowed LTA and operator SMRT to perform additional tests to fine-tune the new signalling systems' software and hardware since last April, he noted, before the new system was implemented on the rail lines.

Mr Chua also made the point that there are many versions of the SelTrac system built by Thales.

"No two CBTC systems are identical as they need to be custom-built," he said. "Each metro uses a unique system architecture, with software logic that is customised for the local environment and infrastructural conditions."

He added that the SelTrac system has been in existence for many decades, and has been adapted for 86 metro lines in 40 different countries, including multiple lines in London, Shanghai, Dubai, Toronto and Vancouver.

Mr Chua also said that the North-South and East-West lines have seen reliability improve, with Mean Kilometres Between Failure (MKBF) going up from 115,000 train-km in 2017 to 556,000 train-km last year.