A man's arm was severed after he fell off a platform and into the path of a train in Sydney last night.

Emergency crews worked for an hour to free the man who fell from Warwick Farm train station in Sydney's west just before 11pm.

The man was rushed to hospital and surgeons are reportedly attempting to reattach his arm.

Police said officers found the 41-year-old man partially trapped under the train with a severed arm.

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"The officers applied first aid to the man, including a makeshift tourniquet, while an operation to free him commenced by Ambulance Paramedics," police said.

Paramedics took him to Liverpool Hospital but his condition is not known at this stage.

Police said investigations were continuing into how the man ended up on the tracks, but it appeared his fall from the platform was an accident.