British Transport Police (BTP) officers were called to West Worthing Station just after 9pm on Tuesday (February 1), following reports of a casualty on the tracks.
The man, who has not been named, was confirmed to be a Southern Rail train driver. Click here to read a tribute from the rail company's managing director
General secretary, Manuel Cortes, said: “It is always heartbreaking to learn of the death of a member of our wider railway community.
"Our union sends our deepest condolences to the family of the driver who was killed.
“No one should die at work, and I have no doubt a full investigation will get to the bottom of what happened in this tragic case.
“Our union takes safety on our railways incredibly seriously and we will continue doing everything we can, working with sister unions and others, to make sure our rail network is safe for workers and passengers alike.”
BTP is working to establish exactly what happened and the Rail Accident Investigation Branch (RAIB) has also begun an investigation.