The MTR Corporation said an incident that led to the suspension of train services on much of the Island Line for part of Wednesday morning was "very rare" and a thorough investigation would be carried out.

At around 5.30am, a rail milling engineering vehicle was returning from routine track maintenance near Wan Chai Station when the crew reported hearing unusual noises, prompting an immediate inspection.

According to KH Lee, deputy director of operations engineering maintenance, staff discovered that some components on the underside of the vehicle had shifted.

He said they also found damage to some equipment alongside the tracks, which led to immediate clean-up and emergency repairs.

"This engineering vehicle, responsible for daily track maintenance, operates nearly every night," he said.

"The situation that occurred this morning is quite rare, and the MTR Corporation takes this incident very seriously."

He added that the corporation will conduct an in-depth investigation and submit a report to the government, with a thorough check of the vehicle planned after service hours.

Train services between Sheung Wan and Quarry Bay were halted to allow for repairs and to ensure safety.

Cheris Lee, deputy director of operating and metro segment, said that workers needed to clear debris and conduct temporary repairs and tests on the signal system before operations could resume.

"Since the repairs required time, we managed to complete the work by around 7.30am, allowing all train services on the Island Line to return to normal," she said.

"The entire section of the Island Line remains safe for train operations and [the incident] has not significantly affected our services."

During the suspension, the MTR deployed extra staff at affected stations and arranged over 30 shuttle buses for passengers.

The corporation reiterated its serious concern over the incident and pledged a full investigation.

Secretary for Transport and Logistics Mable Chan has requested a detailed report from the MTR within one month.