A 22-year-old Oak Flats man who subjected a female commuter to a hellish ordeal on a train between Wollongong and Kiama earlier this year has pleaded guilty to rape.

David John Grooms admitted sexually assaulting the woman on the floor of a rail carriage during the rail journey in March.

He pleaded guilty to sexual intercourse without consent in Wollongong District Court yesterday.

The court was told Grooms had thrown the woman to the ground and raped her while other commuters were just metres away.

On March 30, Grooms had been drinking and possibly smoking cannabis when he met the female victim at Wollongong train station about 6am.

Grooms wandered over to the victim and asked for a cigarette.

He then began harassing her, asking her questions, which she ignored.

When the train pulled up the victim entered the carriage and sat on the upper level. Grooms followed and sat opposite her.

"Hey, move away, there are plenty of seats," the victim said, at which point Grooms moved away.

But when the victim got up and tried to move to the front of the carriage, Grooms grabbed her.

A scuffle followed, during which Grooms grabbed the victim by her hair, slapped her on the right cheek and forced her to the ground. He then raped her.

Meanwhile, two other commuters pressed the panic button alerting two guards. When the train pulled in at the next stop, Grooms backed away from the victim, allowing her to get dressed.

Two guards entered the train and asked Grooms: "What are you doing, mate?" Grooms replied "I'm not doing anything."

The victim was taken to hospital and police tracked down Grooms, who had left the scene, using security camera footage.

He was also identified by witnesses and the victim.

Grooms, who is in custody, will reappear in court for sentencing on February 13.

The rape happened at a time when rail offences - including assaults, thefts, damage to property and liquor offences - were on the rise. After an extensive campaign by the Mercury, then Police Minister David Campbell launched a major offensive aimed at curbing the behaviour.

The initiative involved undercover police, sniffer dogs, highway patrol officers and general duties officers from the Wollongong, Lake Illawarra and Shoalhaven local area commands, as well as transit officers.