A lawmaker has questioned why the MTR Corporation had to shut down the entire Tseung Kwan O line last Thursday, when power and signalling failures halted services for five hours starting from the evening rush hour.

Roundtable's Michael Tien quoted the railway company as telling him that it couldn't switch to manual operation and use the one available track for trains to go both ways because the section was still running on the old signalling system.

Tien, who chaired the Kowloon–Canton Railway Corporation before the merger of the two rail companies, said he didn't find the explanation acceptable.

"When you look at the past two, three years, service disruptions didn't affect the whole Tsuen Wan and Island lines – with both of them also running on the old signalling system," he told RTHK's In the Chamber programme.

"They just switched to manual.

"The design is that, once the automated system isn't safe enough, you can go manual so that the trains would go at a slower speed."

The government has said the MTR has to spend more than HK$19 million on passenger rebates because of the service disruption.

As a result of the Tseung Kwan O stoppage and two others earlier in February and April, the MTRC will be having a day of half-priced fares on a Saturday or Sunday.

Tien said he found it "a bit funny" that passengers across the entire network would benefit, when it was mostly those taking the Tseung Kwan O line trains that bore the brunt of the shutdown.