A young man has died after he was hit by a train in Sydney’s south today, in what police are treating as "misadventure".

Shortly before 2.50pm emergency services were called to Riverwood Railway Station after the young man was hit.

There was a heavy police presence at the scene as officers determine why the victim was in the rail corridor at the time he was hit.

A man has died while reportedly spray painting in rail corridor. (Supplied)

It is believed the man was with another person spray painting a stationary train carriage when he stepped backwards into the path of an express train.

A crime scene was set up at the station to determine the circumstances.

There were major delays for commuters, with buses replacing trains between Riverwood and Kingsgrove earlier this afternoon but trains in the area have since returned to normal.

Trains heading out of the city were not affected by the incident however state transit advised commuters to expect delays on the airport lines.

A crime scene was established and train delays are continuing. (Supplied)