A sagged overhead cable coming into contact with passing trains was to blame for malfunctions in both the power supply and signalling systems that triggered a five-hour service suspension along the Tseung Kwan O line last month, Hong Kong's railway operator revealed on Friday.

The MTR Corporation also vowed to bolster the "overall resilience of the railway network" and strengthen response capabilities, in a report to the government detailing the cause of the breakdown.

Services along the line were disrupted on May 22 during the evening rush hour, leaving tens of thousands of commuters stranded and scrambling to find ways to get home.

In the report, the MTR said the traction power to the overhead line between North Point and Yau Tong stations was interrupted due to a short circuit at around 5.14pm that day, while the signalling system of the Tsueng Kwan O line failed at about the same time.

There was intermittent tripping during the repair process when different trains passed by, the report said.

The MTR said the power system tripping was "caused by a sagged overhead line jumper wire" that resulted in flashover with the roof of trains when they were running at a normal speed.

"The power current surge during the flashover had activated the protection by tripping of the respective [direct current circuit breakers] and also caused interference to the signalling system of the [Tseung Kwan O line]," the report said.

To follow up on the incident, the MTR pledged to change the overhead line design at the line's floodgate sections, introduce additional maintenance guidelines and necessary staff training, and use innovation and technology to facilitate fault diagnosis and recovery.

The railway operator also said it's putting into place measures to reinforce its asset management and maintenance regime, including an immediate one-off inspection of targeted critical assets, and the formulation of "last resort" measures to keep services running.

In receiving the report, Secretary for Transport and Logistics Mable Chan urged the MTR management to work together with all staff members to implement measures set out.

"The [MTR Corporation] must proactively address potential issues before they arise, enhance the safety of its railway systems and accelerate the application of innovative technologies to realise predictive maintenance for fortifying the overall resilience of the railway system," Chan said.

MTR bosses should "strengthen the sense of accountability of staff from different ranks", she added.