Emergency services worked for more than an hour to free the 41-year-old man, who was hit by the train at Warwick Farm station at 10.50pm on Tuesday.

A NSW Police spokesman said the circumstances of how he came to be on the tracks had not been confirmed, but initial investigations suggested he "fell from the platform unintentionally".

Police arrived to find the man's left arm had been severed and he was lying under the train, which had come to a stop at the platform.

Officers applied a tourniquet to stop the bleeding, while police, NSW Ambulance paramedics and Fire and Rescue NSW officers carried out an hour-long operation to free him.

He is understood to have been conscious throughout the ordeal.

The police spokesman said the man was taken to Liverpool Hospital, where he remained on Wednesday morning.

He is expected to undergo surgery in an attempt to reattach his limb, which was recovered.

Train services between Villawood and Liverpool on the T3 line, and between Fairfield and Liverpool on the T2 line, were suspended while the rescue operation took place.

Sydney Trains reported the lines were operating again with a good service on Wednesday morning.

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