Hong Kong is investigating its worst train delay in the public transport system’s history, after a signalling failure of six hours left thousands of commuters stranded on Tuesday.

MTR managing director Jacob Kam Chak-pui described the incident as so unprecedented that outside experts were being brought in. The investigation should take about two months, he said.

“Since this signalling system came into operation in the late 90s, we have never encountered this unusual situation,” Kam told reporters, according to the South China Morning Post. “Not even the design and maintenance handbook mentions such a scenario.”

The city’s rail operator said it had to operate three major lines, along with a fourth line, manually. Instead of running every two to three minutes, trains were running at 12 to 15-minute intervals, according to the MTR. Regular service of the metro system resumed around midday.

The failure, a rarity for Hong Kong, often cited as having one of the world’s best public transport systems, threw the city into chaos. Commuters, used to the efficiency of their rail system were especially frustrated. Trains usually run on schedule 99.9% of the time, according to the operator.

Commuters waited on packed station platforms often for more than an hour for space on a train. To manage crowds, some station operators turned off escalators leading to the platforms.

Long queues formed at bus stations and taxi stand, as people abandoned the trains for other ways to get to work. Residents complained about taxis passing by without picking them up and Ubers charging higher fares.

“I’ve been waiting for like 40 minutes and I still can’t hop on a train, it’s so annoying,” a female passenger waiting at Wong Tai Sin station, in Kowloon, told RTHK.

Hong Kong’s chief executive Carrie Lam asked employers to be considerate of delayed staff, while the education secretary appealed to schools not to penalise late students.

The MTR system, which opened in 1979, is often cited as a model for other public transport systems.

The semi-private company turns a profit and keeps fares low, with fares in 2017 ranging from HK$4 to $59.5 ($0.50 to $7.6). Much of the city revolves around the transport network, with malls built above and around stations, and apartments and community facilities nearby.

Shares in the company, listed in Hong Kong, fell 0.3% by midday Tuesday, after the train delays, and were still down 0.52% on Wednesday. The Hong Kong government is the company’s biggest shareholder.

Observers said Tuesday’s signal failure could have been the result of “buttons getting old” or systems upgrades being done. Officials conceded that they did not know. Lam said: “I don’t know what today’s issue is – is it mechanical? Or manpower?”