Singapore has engaged an expert from Japan's Meidensha Corp. to help investigate the cause of a major disruption to services on the city-state's two main subway lines earlier this month, the Land Transport Authority said Tuesday.

The traction power expert from the Tokyo-based company, which designed and installed the power supply systems on North-South Line and East-West Line, will join other experts in conducting an independent assessment of the two lines following the July 7 train disruption, it said in a statement.

The disruption, which occurred on a Tuesday evening on July 7 for about three hours and affected an estimated 250,000 commuters, has been described by the local media as possibly the worst public transport disruption Singapore has experienced.

Prime Minister Lee Hsien Loong has expressed concern over the problem because the root cause has not been identified and may recur.

Desmond Quek, chief executive officer of rail operator SMRT Corp., apologized Wednesday for the disruption and said his company is committed to improving train reliability.

The experts "will focus on identifying the root cause of the power trip and proposing areas for improvement to prevent recurrence" and "provide recommendations to enhance the overall system and its maintenance," the LTA said.

With the sky-high cost of car ownership due to the government's policies to prevent traffic congestion, the commuter train system is a vital backbone to Singapore's public transport system.

But it has been hit by a slew of train disruptions in recent years. Last year, SMRT was fined a total of 1.6 million Singapore dollars ($1.17 million) for several rail disruptions in 2014 and 2013.

Meidensha is also the contractor providing the power supply system for the northern stretch of Singapore's Thomson-East Coast Line.