A man, who police believe to be a graffiti vandal, has died after being struck by a train in Sydney's south overnight.

Emergency services were called to Kingsgrove train station after receiving reports that a man had been found unresponsive on the tracks late on Sunday evening.

Police accessed the rail line through a security gate and found the man, believed to be aged in his mid-20s, with a mask and spray cans, next to one of the carriages.

Police believe the man had been vandalising another carriage when he ran across the tracks and was hit by a westbound train.

It is not known how the man was able to access the line, which is surrounded by razor-sharp, barbed-wire fencing.

All southbound trains were suspended between Turella and Macarthur while forensic officers combed the site for evidence.

City Rail said all train services, including the T2 line to Sydney Airport, were now operating as normal.

Detective Inspector Paul Simpkins said it was a solemn reminder for people not to enter the railway corridor.

"The circumstances of these events does have some impact on others, the train drivers and emergency services," he said.

"Again we stress to anyone considering entering a railway corridor not to do it. You're gambling with your life."

The man is yet to be formally identified and a report is being prepared for the coroner.

Sydney Trains said it was unable to comment on the man's death as there was now a police investigation underway.