(AFP) - Salvage teams began removing mangled train carriages on Saturday after Taiwan's worst rail disaster in decades killed at least 50 people, as flags flew half-mast across an island plunged into mourning.

Officials said Friday's devastating collision was caused when a parked railway maintenance vehicle slipped down an embankment and onto the tracks.

A train packed with as many as 500 people at the start of a long holiday weekend then hit the truck just as it entered a narrow tunnel near the eastern coastal city of Hualien.

Prosecutors said they are seeking an arrest warrant for the truck driver who may have failed to secure the parking break.

Rescuers described an appalling scene as they rushed into the tunnel and found the front of the train pulverised into a twisted mesh of metal.

"Car number eight had the most serious injuries and number of deaths," rescue worker Chang Zi-chen told reporters on Saturday, referring to the most forward passenger car.

"Basically more than half of the carriage was split open and bodies were all piled up together."

Specialist teams spent hours extracting victims and survivors on Friday.

On Saturday, focus shifted to removing carriages now blocking one half of the sole train line down Taiwan's remote and mountainous eastern coastline.

An AFP reporter at the scene said the most heavily damaged carriages inside the tunnel had yet to be extracted.

Rescuers said further bodies might still be inside the wreckage.

- 'Really devastating' -

The Interior Ministry ordered all flags to be lowered to half-mast for three days while President Tsai Ing-wen visited the wounded in Hualien's hospitals.

"Government agencies are making an all-out effort in the hope of minimising the impact of the disaster so the deceased can rest in peace and the injured can recover soon," she told reporters.

Friday morning's crash took place at the start of the Tomb Sweeping Festival, a four-day public holiday when many Taiwanese return to villages to tidy the graves of their ancestors.

More than 175 people were rushed to hospital. A French national was among the dead.

Survivors gave terrifying testimony of their ordeal inside the train after the crash.

Many of those on board were standing in the aisles because the route was so busy with those leaving the capital Taipei and heading to their home villages.

"I saw bodies and body parts all over the place, it's really devastating," a man surnamed Lo told the Apple Daily newspaper.

"Humans are fragile and their lives are gone all of a sudden."

Morgues in Hualien operated through the night preparing bodies for devastated family members.

Officials have warned that the death toll could rise because some body parts have yet to be properly identified.

Investigators are focusing on how the maintenance truck could have slipped onto the tracks.

The driver, who has been questioned by prosecutors, was part of a team that conducts regular landslide checks on the mountainous route.

Officials said they think he may have failed to properly engage the parking brake.

Apple Daily reported that prosecutors had also raided the offices of the company contracted to do the trackside maintenance work.

- Tourist draw -

Taiwan's eastern railway line is a popular tourist draw down its 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.

Friday's crash took place near two of the most famous landmarks on the eastern shoreline -- the Tarako Gorge and dramatic Qingshui Cliffs.

A world-class bullet train system also serves the heavily populated western side of the island.

Friday's crash looks set to be one of Taiwan's worst railway accidents on record.

The last major train derailment in Taiwan was in 2018 and left 18 people dead on the same eastern line.

That crash was the island's worst since 1991, when 30 passengers were killed and 112 injured after two trains collided in Miaoli.

Other major crashes that killed dozens have taken place in 1981, 1978 and 1961.

Taiwan's most deadly rail disaster on record was in 1948 when a train caught fire and 64 people perished.

© Agence France-Presse