Sydney Trains have revealed the quick-thinking cooperation between control room staff, HAZMAT crews and police that helped them nab a man who allegedly set off capsicum spray in Central Station.

Control room staff first noticed passengers covering their mouths and rubbing their eyes in a tunnel at Central Station in the July emergency, before one person collapsed.

Equipped with 10,000 cameras across the rail network, the control room is able to easily identify trouble and dispatch assistance or contact emergency services.

Police were immediately alerted to reports of a strong smell and burning sensation, and the response team used footage to identify a group of four young men in the area.

The group of men were spotted on a platform. (9NEWS)

Within two hours, the group had been arrested after being spotted on a platform.

“The people who work on the cameras there, real-time, looking at the footage, become very attuned to picking up unusual behaviour, and remarkably they picked up these four characters very quickly,” NSW Police Force Chief Superintendent Donna Adney said.

“Over that period of time, we noticed they were on a certain train… we guided police back to that train which arrived back at Central Station,” Sydney Trains chief executive Howard Collins said.

Hazardous materials crews were called in to evacuate the worst-affected parts of the station and some trains.

A 19-year-old man was ultimately charged over possessing capsicum spray.