But he conceded the removal posed challenges. “It is clouded with uncertainties,” he said, stopping short of issuing a guarantee.

The incident dealt another blow to the embattled operator, which has been targeted in recent weeks by radical anti-government protesters who have vandalised nearly half of its 91 stations. The wave of anger at the MTR Corp followed multiple incidents of clashes between protesters and police in stations and trains, as well as a mob attack on protesters and commuters in Yuen Long.

Critics accused the rail giant of bowing down to Beijing by closing stations as demonstrations began.

Chairman Rex Auyeung Pak-kuen acknowledged the derailment case was another “negative impact”, on top of construction scandals that had plagued the company since last year.

“That’s why I demanded that management find out the truth as soon as possible ... So that the public will know we put safety as our first priority,” he said.

Calling the incident “extremely serious”, Secretary for Transport and Housing Frank Chan Fan, who was on the scene from about 10.30am, pledged a full official investigation, while MTR managers vowed to set up an investigative panel of their own.

Alfred Sit Wing-hang, director of the Electrical and Mechanical Services Department, which would investigate the accident on the government’s behalf, said he hoped the probe would be done in three to six months.

Extra buses laid on for rush hour commuters after derailment causes disruption

“We are striving to conduct an independent, comprehensive and speedy investigation in order to get to the bottom of the truth,” he said.

The MTR Corp was asked by the media about possible causes ranging from cracks on train tracks, to replacement work carried out the night before and even sabotage by protesters.

The rail firm and the government – a major shareholder – said they would not rule out any possibilities, but police, who also launched a criminal probe, found no suspicious objects at the scene as of Tuesday evening.

It was the second time in a year a major operational incident had hit the company. In March, two trains collided during a trial run outside service hours. While there were no passengers involved, the incident crippled services in the heart of the city.

The last time that a train carrying passengers derailed was at Kowloon Bay station in 1994.

In Tuesday’s incident, eight of the about 500 passengers on board were injured, with one man and four women later sent to hospital with a sore neck or shoulders, or cuts to their arms, according to the fire service.

The services of the East Rail line – the city’s only local line connecting to the border with mainland China – were disrupted for a good part of the day, with trains running less frequently at unaffected stations until 4.30pm, when services at those sites returned to normal.

The disruption spread to nearby roads, as a mobile crane, called to the scene to remove the carriages, blocked a lane leading to the already packed Cross-Harbour Tunnel.

MTR operations director Lau apologised on behalf of the company, while Auyeung visited injured passengers at Queen Elizabeth Hospital in Yau Ma Tei.

“For the inconvenience caused to our passengers due to this incident ... we offer our sincere apology. For our injured passengers, we extend our deepest sympathy,” Lau said.

It was understood the derailment happened at a diverting point, where trains are directed to different platforms or other locations, their direction and speed controlled by computer.

According to documents seen by the Post, a section of rail between the two stations was scheduled to be replaced during Tuesday’s early hours. An insider source confirmed that the work went ahead, and was in the area where the derailment happened.

Lau confirmed the work happened near the site of the accident, but said it was not on the same stretch of track. He also confirmed three cracks had been found on the track, two of which were about 30mm wide each.

MTR cancels overnight service for Mid-Autumn Festival over safety fears

“We can’t be sure if the cracks were the cause of the derailing,” he said, noting earlier that it was not discernible yet if they were there before or after the incident. He also added he saw no link between the derailment and the fact that the train was heading into a diverting point.

The train in question began service in 1983, and it was refurbished in around 1997, Lau said.

Pledging full cooperation, Lau said the MTR Corp would set up an investigative panel of external experts, which would also conduct an in-depth probe.

He said ground staff had conducted the first round of checks before six unaffected carriages and one derailed car were towed away.

But he added that staff had difficulties removing the rest of the two damaged cars to be sent back to the depot for further examination. There will be further scouring for evidence after the complete clearance of the site.

Staff would only begin repair work after all this was completed, according to Lau.

Transport minister Chan was asked about the possibility the derailment was orchestrated by protesters.

“We do not rule out any causes,” he said, adding that a full investigation was needed first.

Lau said all trains had to be removed before they could determine if there had been obstructions. Police would also carry out their probe once the site was cleared.

A female passenger described the moment of derailment as an explosion, before she was stretchered away from Hung Hom station.

A man, who gave his surname as Kwok, was seen at the station in a wheelchair. He said he had knocked his head when the train derailed.

Kwok told reporters he heard a strange noise, after which the carriage began shaking and veering from side to side.

The carriage he was in held 40 to 50 passengers, mostly women, and at least one child. Kwok said he helped them leave before he was tended to by first-aid workers.

Another man, who was on the carriage next to one that broke off, said there was no announcement or guidance from the train conductor or MTR staff and passengers made the decision to leave the carriage by themselves as dust, smoke and debris began filling the air.

Images circulating online showed passengers leaving the carriage, which stood across two sets of tracks, through a damaged train door. They were seen walking along the tracks to safety.

Lawmaker Lam Cheuk-ting urged the MTR Corp to thoroughly investigate the incident.

“It is fortunate there have not been any serious injuries or casualties, but if the train was going at high speed, the situation could have been worse,” Lam said, speaking at a briefing outside the Legislative Council building.

Pan-democrat lawmaker Wu Chi-wai, who was also at the briefing, said the MTR Corp should make sure the signalling system was working in sync with the track diversion system to prevent further incidents.

Trains on the West Rail line were also affected, but normal service resumed by 11.40am.

The MTR Corp said 19 train services between Hung Hom and the mainland would be cancelled on Tuesday, 18 to and from Guangzhou and one from Beijing.