A man who was struck and killed by a train at a station in Sydney's south is believed to have been spraying graffiti at the time.

Authorities were called to Riverwood Station at 2:50pm today following reports a man had been killed by a train.

It is believed two men were on the tracks but the other man was unharmed.

Police said they were treating the incident as "misadventure" and were investigating how and why the men were on the train tracks.

There were heavy delays for commuters heading towards the city after the incident.

Buses replaced trains between Revesby and Kingsgrove for a number of hours, but services have since returned to normal.

Outbound trains were unaffected.