The AP reported that a truck somehow fell off a cliff and landed directly in front of the tunnel.
At least 34 people have died and dozens have been injured in a train crash in Taiwan.
The official Central News Agency said a truck whose handbrake was not engaged was suspected of sliding off a sloping road into the path of the train, and that police had taken in the driver for questioning.
The crash, north of the eastern city of Hualien, killed the driver of the train carrying many tourists and people heading home at the start of a long weekend traditional holiday to tend to family graves.
The train had only partially emerged from a tunnel, and with much of it still inside, many escaping passengers were forced to scramble out of doors and windows and scale the sides of the train to walk along the roof to safety.
"Our train crashed into a truck", one man said in a video aired on Taiwanese television, showing pictures of the wreckage.
Local media reports suggest that while the second and third cars derailed, carriages five to eight have sustained significant damage inside the tunnel, hampering rescue efforts. The inner wall of one carriage was pushed all the way into the adjacent seat.
The National Fire Service confirmed that 48 people were dead and more than 100 injured. No-one was in the truck at the time.
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An investigation has been launched into the crash, and there was no immediate word about any arrests. The train was en route from the northern part of the island to the southeastern city of Taitung.
In a tweet, Taiwanese President Tsai Ing-wen said emergency services "have been fully mobilised to rescue & assist the passengers & railway staff affected".
Weng said the speed of the train at the time of impact was not known. "We will continue to do everything we can to ensure their safety in the wake of this heartbreaking incident".
The accident came at the beginning of a long weekend for the traditional Tomb Sweeping Day holiday.
In 2018, a passenger train with 366 passengers on board derailed in Taiwan's Yilan County, claiming 18 lives and wounding over 160.
Taiwan has suffered the deadliest train crash in its history. He said the crash was the worst-ever in Taiwan.
Taiwanese railway officials will be required to conduct sweeps along other tracks in the system to "prevent this from happening again", Su Tseng-chang, the country's premier, said.
Taiwan's mountainous east coast is a tourist destination.