"People just fell all over each other, on top of one another", a woman who survived the crash told a local television station. "It was terrifying. There were whole families there".
The official Central News Agency said a truck that was "not parked properly" was suspected of sliding into the path of the train.
Many people may have been standing because the train was so full. Many passengers were tourists or people traveling to see family ahead of a yearly religious holiday.
A French national was among those killed while two Japanese and one Macau resident were injured.
The train, from the capital Taipei to Taitung, was carrying people travelling for a long-weekend annual holiday.
The accident occurred on Taiwan's eastern railway line around 9:30am (0130 GMT).
Feng said the manager of the site, which was stabilising the mountainside to prevent landslides, visited around 9am local time and stopped his truck in front of the site office.
And in 2003, 17 died and 156 were injured after a train on the Alishan mountain railway plunged into a chasm at the side of the track.
'Is everyone out in carriage four?' a lady is heard shouting from inside the tunnel, in footage provided by the fire department.
Survivors described their terror as the train hit the truck and stopped.
"It felt like there was a sudden violent jolt and I found myself falling to the floor", one female survivor told Taiwan's UDN. "It was all so chaotic".
Part of the train was situated outside the tunnel, and those passengers in carriages still in the tunnel had to be led to safety, Taiwan's railway administration said.
At least 36 people were killed and 72 injured when a Taiwan train derailed in a tunnel on Friday after apparently hitting a truck.
Images showed passengers gathering suitcases and bags in a tilted, derailed carriage and others walking along the tracks littered with wreckage.
Numerous passengers would have been traveling for the first day of the four-day Qingming, or Tomb Sweeping Festival - an annual pilgrimage to the gravesites of ancestors.
Taiwan's mountainous east coast is popular with tourists. The line connecting Taipei with Hualien was opened only in 1979.
Taiwan's state-owned railways are generally reliable and efficient, but have had a patchy safety record over the years.
Taiwan's last major rail crash was in October 2018 when an express train derailed while rounding a tight corner on the northeast coast, killing at least 18 people and injuring approximately 175.
In 1948, 64 people were estimated to have died when a train caught fire in northern Taiwan.