The same day a group of young men jumped onto the tracks at Corrimal railway station as a train approached, police had already received reports of another man trespassing on the line at Thirroul.
Wollongong police Inspector Don Faulds said police had been notified of a man trespassing about 3pm last Friday.
He said people risking their lives and using the train line as a shortcut was common, and urged those doing it to reconsider.
"It's dangerous, it's illegal and if we catch them we will give them a fine on the spot," Insp Faulds said.
According to Sydney Trains, the maximum fine for an individual trespassing on the tracks is $5500.
Images and video showing two young men running across the tracks in front of an oncoming train published yesterday by the Illawarra Mercury online and in print have drawn a strong response from the community.
One online commentator said: "These kids should be arrested and made to watch what really happens to someone when they are killed by a train."
The incident happened between 9.30pm and 10pm on Friday.
Hayley Jones was waiting at the platform when she witnessed the incident.
She said seeing the young men risk their lives made her hesitant about catching the train again.
"My heart was pounding, it was so scary," Ms Jones said.
"There were three or four that jumped off the platform and one guy at the last second jumped the line. Then the driver of the train came out and had a big talk with them."
A NSW TrainLink spokeswoman said since September last year, an average of one trespass per month had been reported at Corrimal station.
"Trespassers are taking their lives in their hands by entering the rail corridor and their behaviour also has a significant effect on others," the spokeswoman said. "If a trespass is reported, trains must be stopped and customers are delayed and the psychological impact can be severe for customers, train drivers and other staff should a train hit someone.
"NSW Police officers patrol the Sydney and NSW Trains networks and officers work closely with our staff to stop trespassers," she said.
NSW Trains had reviewed CCTV footage of the incident at Corrimal station and was working with the Police Transport Command to find the offender, she said.
The footage has been passed on to police.