Lawmaker Gary Zhang said he was highly unsatisfied over MTR Corporation's having taken more than five hours to resume services on its Tseung Kwan O line in the wake of Thursday's evening rush hour suspension that left thousands of people stranded.

The rush-hour suspension of services along the entire Tseung Kwan O line has been blamed on multiple system failures, including signalling problems and a power supply trip.

Speaking on an RTHK programme on Friday, Zhang, chairman of the Legislative Counci's railway subcommittee, pointed out that the MTR has already experienced multiple serious service delays or suspensions this year.

"For example, like in February and also in April, there were two very serious incidents involving engineering trains on the East Rail line, each causing several hours of delay," he said.

"So I would say this year the MTR service performance is highly unsatisfactory."

Zhang, a former MTRC engineer, said that even with daily inspections, overhead cable components – which supply electricity to train systems – can still fail.

He urged the railway operator to expedite the installation of real-time sensors along the tracks to help engineers understand the cause of problems and shorten repair times.

From his understanding, he said, the MTR may be experimenting with such technology, but they have yet to be fully implemented across the network.

Increasing staffing levels for maintenance teams could also help, Zhang noted, saying that according to his knowledge, the MTR has been experiencing persistent staff shortages in recent years.

Zhang also said he went to Tseung Kwan O station for on-site observations on Thursday night and noticed many passengers stranded on the platform, waiting for train services to resume.

This shows MTR staff need further training, he said, particularly in emergency response or contingency handling capabilities.

Regarding the possible penalties to be imposed on the MTR, Zhang said that based on his preliminary calculations, the railway giant could face a fine of about HK$18 million.