The Puyuma express ran off the tracks late Sunday afternoon as it went around a bend, throwing train cars into a zig-zag pattern with five left lying on their sides. There was no immediate word on the cause. Survivors interviewed by Taiwan's official Central News Agency said the driver had applied emergency brakes multiple times before the train derailed.
Most of the deaths were in the first car, and it was unclear whether other people were trapped in the train, according to a government spokesman, who spoke on the customary condition of anonymity.
Some passengers were crushed to death, Ministry of National Defense spokesman Chen Chung-chi said. "Their train car turned over. They were crushed, so they died right away," Chen said.
Taiwanese President Tsai Ing-wen visited the crash site in Yilan County in the island's northeast early Monday morning and said her government had instructed the authorities to work quickly to investigate the cause of the derailment.
The train had been carrying more than 360 passengers from a suburb of Taipei in the north to Taitung, a city on Taiwan's southeast coast.
Earlier, the government put the death toll as high as 22, but the National Fire Agency, citing the Cabinet spokesman's office, later reduced that figure and blamed a miscalculation.
Local television reports said passengers tried to escape through windows and bystanders gathered to help before rescuers arrived.
One of the eight cars tipped at about a 75-degree angle, with its entire right side destroyed.
Fearing people may be trapped beneath the car, firefighters with lights on their hard hats peered underneath as a crane prepared to upend it. The firefighters were joined by soldiers and Buddhist charity workers who gathered on both sides of the tracks.
Soldiers removed bodies to identify them, but nightfall complicated the rescue work.
On a live feed provided by Taiwan's United Daily News, rescuers could be seen carrying what appeared to be a body wrapped in white plastic away from the site.
Searchers walked through an upright car with flashlights. The search-and-rescue work was to continue until early Monday to make sure everyone aboard was accounted for, Premier William Lai told reporters shortly after midnight.
"The underlying cause should be investigated to the maximum extent to avoid anything like this happening in the future," Lai said. "We will make the whole thing transparent."
Ensuring that rail traffic goes back to normal is also a priority, he said.
Most people who were seriously hurt suffered head injuries and one was bleeding internally, said Lin Chih-min, deputy director of Luodong Boai Hospital, where four people were in intensive care. The hospital had treated 65 people total.
The Puyuma was launched in 2013 to handle the rugged topography of Taiwan's east coast. It is distinct from the high-speed rail that runs on the west coast. The Puyuma trains travel up to 150 kilometers (93 miles) per hour, faster than any other in Taiwan except for the high-speed rail.
The train that derailed had its most recent inspection and major maintenance work in 2017, Taiwan Railways Administration Director Lu Chie-shen said at a televised news conference.
Sunday's derailment was at least the third deadly rail accident in Taiwan since 2003.
A tourist train overturned in the southern mountains in 2011 after a large tree fell onto the rails. Five Chinese visitors were killed.
A train on a test run ignored a stop sign and crashed into another train in northeastern Taiwan in 2007. Five people were killed.
And in 2003, a train derailed near a mountain resort, killing 17 people. Investigators blamed brake failure.
The Taiwan Railways Administration bought the Puyama from Japan's Nippon Sharyo in 2011 in a 30 billion yen ($260 million) deal. The eight-car trains are designed to tilt when going around curves and have a maximum speed of 150 kph. The company's announcement at the time said the trains were part of a 1.45 trillion Taiwan dollar ($46 billion) upgrade of the line along Taiwan's east coast.
Japan has been competing assiduously with China, South Korea and European railway providers to win overseas contracts as its already saturated home market shrinks.