TAIPEI (Reuters) - Taiwan authorities said on Friday a 55-year-old man, most likely acting alone, was the key suspect in a bombing on a train that injured 25 passengers.

Police said the attack was carried out with a suspected homemade pipe bomb and unlikely to be terror-related.

Liu Po-liang, commissioner of the Criminal Investigation Bureau, named the suspect as Lin Ying-chang, who was among the injured in the blast late on Thursday and was in critical condition in hospital.

Authorities described the explosive device as a steel tube 47 cm (19 inches)-long, filled with pyrotechnic gunpowder.

Based on evidence including Lin's injuries and clothing and surveillance video of his movements before boarding the train, he likely acted alone, Liu told a news conference.

Surveillance video shown at the news conference showed Lin carrying a long red bag that investigators had found in one of the train cars, which police said may have been used to transport the device.

"We have locked on Lin as the suspect," Liu said. "Currently it doesn't appear that he had accomplices."

Liu said it might be a few days before authorities can question Lin due to his injuries. It remained unclear how the device was set off, he said.

Premier Lin Chuan said on television the attack appeared to be a deliberate "act of malice". The bomb went off just before the train entered a station in Taipei, the capital.Television showed people with burned limbs and faces being taken to hospital.

"Our initial investigation has ruled out terror," Wang Bao-chang of Taiwan's National Police Agency told reporters earlier, adding there had been no claim of responsibility.

(Reporting by J.R. Wu; Editing by Andrew Roche)