The train driver saw the man about 300m away and attempted to alert him by blasting his horn. But the man continued to walk backwards and stepped into the path of the train.
The train had five passenger carriages and came to a stop as soon as possible, police said.
Witnesses had confirmed the bells and level crossing 20m away were working.
Police spokeswoman Kimberley Mathews said the man had been wearing a high-visibility outfit.
The people he had been talking to were doing construction work in the area, she said.
There was nothing to indicate he had intentionally stepped in front of the train and all indications at this stage were that it was a tragic accident.
Rail operator Transdev said the Southern Line had reopened and train services would resume shortly.
It expected train services to be back on schedule by this evening.