The train, heading to the city of Taitung, was carrying around 350 people when it hit a truck in its path, sending carriages crashing into the walls of the tunnel, according to reports.
Dozens were wounded and more than 70 passengers were trapped inside the hard-to-reach crash site, the national fire department said.
Images of the crash show the carriages smashed inside the tunnel, with wreckage littering the tracks. Passengers were pulled out as rescue teams helped people get on top of the train to safety. Others who were able walked through the tunnel with their luggage.
The back of the train appears lodged inside the tunnel, with no signs of easy entry for rescue efforts.
“We will continue to do everything we can to ensure their safety in the wake of this heartbreaking incident,” Tsai said in a post to her official account.
The derailment happened at the beginning of Tomb Sweeping Day, a national holiday in Taiwan.
Over a long weekend, families visit their ancestors’ graves to clean tombstones and visit popular sites. Hualien, which is just north of where the crash took place, is near Taiwan’s famous Taroko Gorge.
Major railway accidents are rare in Taiwan, whose high-speed rail covers much of the country.
The last incident was in 2018, when a passenger train derailed in Yilan County and killed 18 people. But the death toll from Friday’s accident was the worst since 1991, when 30 people died after two passenger trains collided in Taiwan’s western Miaoli County.