Initial inquiries suggest the man in his 50s had a medical episode that caused him to fall onto the tracks and into the path of an oncoming train.

Shortly before 3.30pm police were called to the station, where they found a man trapped beneath a train.

He was treated by paramedics for serious injuries to his foot, before being taken to St Vincent's Hospital, where he remains in a serious but stable condition.

Mehrial Parsa was standing on the platform a few metres from where the incident occurred.

"My head was turned away when it happened, but suddenly I just hear people screaming 'stop, stop, stop'."

She says the train driver slammed on the brakes and stopped the train about a third of the way into the platform.

"He jumped out and ran over to where the person was trapped. He was absolutely devastated, holding his head in his arms," Ms Parsa said.

Five to 10 station staff ran over to the man, before ushering "devastated" commuters away from the platform, she said.

Train services between Wynyard and North Sydney were initially suspended. Service resumed around 4.45pm, however travel times may be affected.

Commuters can check current service updates.

The Transport Management Centre said passengers planning to travel from North Sydney to Wynyard could use their train tickets to catch buses.

"Passengers travelling on the T1 Northern Line and T1 Western Line are also advised to allow additional travel time, listen to announcements and check indicator boards at stations."