The 26-year-old man, nicknamed "A Hao", was arrested Wednesday and is considered the second suspect in Taiwan's worst train derailment in seven decades in Hualien City on the east coast that killed 50 people and injured more than 200 others last Friday, Taiwan News reported.
The Vietnamese suspect has lived in Taiwan illegally for four to five years.
Lee Yi-hsiang, the main suspect, was detained last Sunday.
An express train carrying 500 passengers hit the crane truck, operated by Lee that had slid down a bank beside the track from a building site.
Railway authorities suspect the truck was parked without the emergency brake engaged.
While prosecutors were investigating the accident, they discovered surveillance camera footage showing another man with him in the crane truck loaded with waste tires. Prosecutors identified the other man as an illegal worker from Vietnam, Japan's Kyodo News reported.
Lee was seen sitting behind the wheel while the Vietnamese was sitting in the passenger seat.
Both Lee and A Hao were detained for further questioning on suspicion of causing death by negligence, according to Hualien District Court on Thursday.
After questioning Hao for about five hours, prosecutors determined "the Vietnamese man was a flight risk and could try to collude with other defendants and destroy evidence," Taiwan News reported.
Therefore, prosecutors applied to have Hao held incommunicado.