NSW Police are making inquiries into a video circulating online of an incident on a train in Sydney.

Video has surfaced online of a young man threatening to “crack” the head of an elderly man who had told him to be quiet on a Sydney train before other passengers jumped into the fray.

The incident reportedly occurred on a train from Bondi Junction to Central station during daylight hours on Wednesday, with at least a dozen other people visible in the footage.

“Shut up,” the older man, wearing a white hat, says sitting in the row in front of the youth.

“Make me, make me … what the f**k are you gonna do?” the younger man replies.

The brunette youth, wearing a T-shirt with bright green and red stripes, jumps to his feet.

The older man also gets to his feet but is encouraged by a woman next to him to sit down.

“Huh? You wanna arc up to me d**khead?” the young man yells.

“Arc up to me d**khead and I’ll crack you.”

The situation, on the bottom of one of Sydney’s two-level trains, then takes a dramatic turn.

A man in a suit gets out of his seat near the front of the carriage and storms towards the argument.

He watches on for a few seconds before he grabs the young man with two hands by the throat and shoves him down into the seat.

“What you say? What you say? Who are you going to crack? Who are you going to crack?” he asks, shaking the younger man.

With a final shove, he tells the youth to “get off the train now” as others repeat the direction.

“Get off the train you donkey,” the man in the suit says.

“You gonna hit an old man you piece of s**t? Why don’t you hit me? Why don’t you hit me? You b*tch, you’re a f**king b*tch”.

The video lasts 59 seconds and it’s not known how the situation ended.

News.com.au contacted police for further details.

“NSW Police are aware of the video circulating online of an incident on a train and are making inquiries,” a spokeswoman said on Thursday morning.

“No further information is available at this stage.”

Sydney Trains did not make any comment due to it being a police matter.