TAIPEI, Taiwan — The train, cruising in and out of mountain tunnels along Taiwan’s east coast, was packed with people rushing to see family and friends on the first day of a long holiday weekend.

Then, according to survivors’ accounts, it was jolted by a heavy crash, flew off the rails and slammed into the walls of a tunnel.

The derailment of the eight-car Taroko Express train on Friday morning was the worst such disaster in Taiwan in four decades, killing at least 51 people, including two train drivers, and injuring around 150 others, the authorities said.

Investigators are still trying to determine why the train crashed as it was traveling from near Taipei to the eastern coastal city of Taitung. But initial reports indicated that it had either collided with a construction vehicle that rolled down a slope onto the track, or was hit by the falling truck just as it passed.

By Friday evening, rescue workers had freed dozens of passengers who had been trapped in the wreckage, but were struggling to get to several train cars that were deep inside the tunnel. Local news footage showed one worker using an electric circular saw to cut through one of the twisted carriages.

Video footage posted online showed rescuers carrying injured passengers out on stretchers as other survivors emerged from the tunnel walking on the roofs of the train’s cars, some rolling suitcases. Several passengers described smashing the windows of the carriages with their luggage to escape.

A passenger surnamed Wu told Taiwan’s official Central News Agency that the last thing he remembered before passing out was a loud crash. When he regained consciousness, the train was shrouded in darkness and he and several passengers used the light from their cellphones to see. They tried to help the other injured survivors, he said, but it took them an hour to find their way out of the train.

“I’m already safe, but I didn’t dare to look at the crash scene,” he said. “Many bodies were lying there.”

The crash occurred around 9:30 a.m. in a tunnel just north of the city of Hualien near Qingshui Cliff, a destination popular among tourists who flock to see towering mountains and crystal-blue waters. Friday was the annual Tomb Sweeping Day holiday, a time when Taiwanese often travel. A railway official told Taiwan’s United Daily News that the train had 374 seats and was near capacity.

The Taroko Express train is one of the fastest to traverse Taiwan’s east coast and typically travels at around 80 miles per hour. In interviews with local news outlets, survivors described the train as being crowded, with many passengers standing during the journey. Some said in video interviews that the carriages they were in had filled with smoke, and that they could see passengers who had been rendered unconscious and trapped.

The death toll makes the train crash one of the worst disasters that Taiwan’s president, Tsai Ing-wen, has faced since she took office in 2016. Within hours of the crash, Ms. Tsai said the government had fully mobilized rescue services. Later, she vowed to conduct a thorough investigation into the cause of the collision.

“We pray for the victims to rest in peace and for the injured to recover as soon as possible,” she said at a news conference late Friday afternoon.

In the last major train accident, in 2018, 18 people were killed and 170 others injured after a train derailed in northeast Taiwan’s Yilan County on a coastal route popular with tourists. Taiwanese investigators later found that the train had been going too fast and that the driver had manually disabled a system designed to prevent it from exceeding safe speeds.

Train accidents are still fairly rare in Taiwan. The last crash of a similar scale took place in 1981, when a train collision in the island’s northwest killed 31 people.

A railway official said they believed the driver of the construction vehicle parked on a slope near the entrance of the tunnel and may have forgotten to engage the emergency brake, causing the truck to roll down and hit the train just as it was passing, according to the Central News Agency. The driver is not believed to have been in the truck at the time.

A cellphone video filmed by a passenger and posted on social media showed what appeared to be a yellow trailer lying on its side next to the derailed train at the entrance to the tunnel.

“Our train crashed into this truck,” said the passenger in the video. He panned the camera to show a grassy slope beside the tunnel. “The truck rolled down, and now the whole train is twisted.” Local media outlets published a photo showing a single truck door lying on the grass.

The police took the operator of the construction vehicle in for questioning, according to a police official in Hualien County who was reached by phone.

Lin Chia-lung, Taiwan’s transportation minister, told reporters at the crash site on Friday that while he had done his best to strengthen accountability and reform the railway system following the 2018 disaster, “clearly the speed and results of the reforms were not enough.”

“I am responsible, and I should take responsibility,” Mr. Lin said.

Wei Yu-ling, secretary-general of Taiwan Rail Union, said in an interview that she expected the government to conduct a thorough investigation into Friday’s crash, which comes not long after a maintenance train hit and killed two railway workers and injured another in Taitung County in eastern Taiwan.

The recent accidents, she said, “exposed the inner problems of the Taiwan Railways Administration from top to bottom.”

Photos of Friday’s crash circulating online indicated the damage was severe. One image posted by United Daily News, a Taiwanese news outlet, showed what appeared to be the train’s mangled control car on its side in the dark tunnel. The train’s conductor told a local television station that he had been on one end of the train when he felt what seemed like the emergency brakes being applied and a sudden jolt.

“Many people were stuck under the chairs and piles of bodies,” a woman surnamed Wu told ET Today, a Taiwanese news station, in a televised interview from the hospital where she had been treated for light injuries. “At the beginning I could hear them crying for help, but then maybe they fell asleep or something. Also I saw many children, so pitiful, so pitiful.”

Most train service along Taiwan’s eastern routes has been suspended until Sunday morning, causing travel delays for many at the start of a long holiday weekend. Tomb Sweeping Day, an ancient Chinese festival also known as Qingming, is a time in which the living pay respect to their ancestors by tidying their graves and burning paper offerings.

A woman who was traveling back home with her husband to sweep the family tombs in Taitung told local reporters at the site of the accident that she had been asleep in the seventh carriage when the train crashed, throwing her to the floor. The woman’s shirt was bloodied, and a plaid scarf had been tied around her head to stem the bleeding.

“We’ve always tried to take the train whenever possible,” she said, as rescue workers in yellow hard hats worked behind her. “We never thought something like this would happen.”

Joy Dong reported from Hong Kong.