The fire department showed a picture of what appeared to be the truck's wreckage lying next to the derailed train, and an aerial image of the end of the train sitting on the track next to a construction site.
"The top priority now is to rescue the stranded people", it said in a statement.
The official Central News Agency said a truck that was "not parked properly" was suspected of sliding into the path of the train.
They told NHK the train hit a construction vehicle, but the exact cause is under investigation.
Taiwan's extensive rail system has undergone substantial upgrades in recent years, particularly with the addition of a high-speed line connecting the capital Taipei with west coast cities to the south.
Pictures published by local news website UDN showed the front of the train inside the tunnel had been pulverised into a twisted mesh of metal. Rescuers are struggling to reach survivors inside cars in the tunnel.
The train had only partially emerged from the tunnel, and with much of it still inside, escaping passengers were forced to scale doors, windows and roofs to reach safety.
Another rescued woman said: "My whole body fell to the floor".
Many people may have been standing because the train was so full.
"Around four people are without life signs", the Central Emergency Operation Center said in a statement, adding rescuers were trying to get to four carriages inside the tunnel that were badly damaged and hard to access.
'We broke the window to climb to the roof of the train to get out, ' she added.
The accident occurred at the start of a long weekend for the traditional Tomb Sweeping Day hospital.
During the festival, people return to ancestral villages to tidy up the graves of their relatives and make offerings.
Taiwan's eastern railway line is usually a popular tourist draw down its dramatic and less populated eastern coastline.
With the help of multiple tunnels and bridges, it winds its way through towering mountains and dramatic gorges before entering the picturesque Huadong Valley.
Taiwan has suffered one of the deadliest train crashes in its history.
In 2018, a Puyuma Express train had derailed in the island's Yilan County, reportedly killing 18 people and injuring 170.
The driver of the eight-carriage train was later charged with negligent homicide. Authorities say at least 41 people have lost their lives and more than 60 are injured.
In 1991, a collision in western Taiwan killed 30 people and injured 112.