The 44 year-old boarded the train at Newcastle and began yelling shortly before arriving at Central station.

A spokesman for NSW Police said the man had been making general threats and had not targeted anyone on board.

A fellow traveller and witness to the threats, who contacted Fairfax Media, said he encountered the man when he boarded the train at Epping and described the man's behaviour as aggressive and erratic.

"He seemed to wanting (sic) to pick a fight with other passengers but many passengers just ignored him," the witness said.

He also said there were about 15 passengers in the lower level of the carriage, where the man was sitting, pointing and trying to peal off stickers that displayed emergency services details such as 000 and Police.

"After failing to do so he was asking passengers to give him a knife or a gun so he could cut it off or shoot it," the witness said.

"After sometime a passenger was on their phone and he walked up to that passenger and told them to get off as he obliviously thought he was being reported, and the passenger got off the phone not wanting a fight with this guy."

The man was arrested at Central and taken to the Day Street police station, where he will undergo a mental health assessment.

Officers from the Police Transport Command and also Fire and Rescue NSW conducted checks of the entire train before deeming it safe to begin a new service to Newcastle.

Do you know more? Email rose.powell@fairfaxmedia.com.au