A man is in a critical condition after his arm was severed when he fell into the path of an oncoming train in Sydney's southwest.

Police say emergency services worked for more than an hour to free the 41-year-old after he apparently fell accidentally from Warwick Farm station platform onto tracks about 10.50pm on Tuesday.

Officers arrived to find him trapped under the train with his arm severed at the elbow. They applied a tourniquet to stop the bleeding until the man was freed.

He has been taken to Liverpool Hospital where he remains in a critical but stable condition. He is expected to undergo surgery to reattach the limb on Wednesday.