At least 50 people died in Friday’s crash, which was caused by a vehicle slipping onto the tracks.
A construction site manager whose lorry slid on to a railway track and led to Taiwan’s worst train disaster in decades has expressed “deep regret”.
Lee Yi-hsiang, 49, said he was “deeply remorseful” and wanted to give his “most sincere apologies”.
His flatbed lorry was parked on an embankment but slipped down it, causing the train to derail on Friday near the city of Hualien.
At least 50 people were killed and more than 200 injured in the crash.
Investigators say CCTV footage from the front carriage showed the train driver had only 6.9 seconds to respond and the train was only 250m (820ft) away from the lorry, not enough time or distance for the driver to stop and avoid the collision.
The probe is now looking into whether Mr Lee failed to set the emergency brake or whether there was a mechanical failure in his vehicle.
He was questioned over the weekend by prosecutors and released on bail, but on Sunday he was taken back into custody because he was deemed a flight risk and had a previous conviction, Taiwanese media reported.
Reading a statement to news crews outside his house, Mr Lee said he would co-operate with crash investigators, and “take the responsibility I should take”. He was then taken away by police.
He was part of a team who regularly inspected Taiwan’s mountainous eastern train line for landslides and other risks. He was also thought to be the flatbed’s operator.
The eight-carriage train was travelling from the capital Taipei to Taitung when it hit the flatbed and crashed inside a tunnel north of Hualien.
The train was packed with people travelling to celebrate a long-weekend holiday, and many of the nearly 500 passengers on board may have been standing because the train was so full.
Some survivors lost their whole families, AFP reports, and Taiwan declared three days of national mourning.
Crews are still slowly and carefully removing the train wreck from the tunnel. There are fears more bodies could be found.
Investigators have been going through the train’s recording devices and CCTV footage from the front carriage, the chairman of the Taiwan Transportation Safety Board told AFP.
“According to the testimonies by some passengers, they heard the horn being sounded and it’s believed the train driver had spotted an object on the track,” Hong Young said.
He added that the train driver, who was among those killed, would have struggled to avoid the crash.
There have been mounting questions over how full the train was, and why there were no barricades on that section of the track.
This lead to Taiwan’s transport minister, Lin Chia-lung, offering his resignation on Sunday. On Facebook, he wrote: “I should have accepted all the criticism over the past few days, but we have not done well enough.”
The government has not accepted his resignation, however, and said he should stay in the position until the investigation was complete.