Witnesses sounded the alarm just after 5pm on Friday when a man in a blue jumper was spotted taking photos of trains while on the tracks near Southern Cross Station.

Police are currently responding at the scene to ensure the safety of the man but the operation has slowed down trains in the CBD.

Metro Trains has reported major delays for passengers on the Alamein, Belgrave, Glen Waverley, Lilydale, Craigieburn, Watergardens, Upfield lines until the issue is resolved.

It comes as an accident has shut down all outbound lanes on the Princes Highway at Avalon.

Emergency services were called to the crash near Avalon Rd just before 5pm after reports of an smash involving multiple vehicles.

An Ambulance Victoria spokesperson said two people were injured but both appeared to be in a stable condition.

Paramedics are treating a woman, believed to be in her 20s, for chest and abdominal pain while a man, also believed to be in his 20s, is being treated for neck pain.

Two Melbourne-bound lanes have also been closed at the scene.