Officers are looking into the matter following The Broadway being flooded with police, ambulances and firefighters today (December 8) at 2.33pm.
According to the British Transport Police (BTP), this was due to reports of a casualty on the tracks.
A London Air Ambulance was seen landing in the area, with several emergency services vehicles parked outside Wimbledon station.
The man was pronounced dead at the scene, the BTP said.
However enquiries are ongoing into the circumstances leading up to the death.
Thameslink trains between Streatham and Sutton have been closed while emergency services were called to the tracks.
South Western Railway trains have also been delayed.
It is expected that major disruption will last all day.
A BTP spokesperson said: “Officers were called to the line in Wimbledon at 2.18pm on 8 December to reports of a casualty on the tracks.
“Paramedics also attended, and sadly a person was pronounced dead at the scene. Enquiries are ongoing into the circumstances leading up to their death.”
If you feel affected by what has happened and wish to find support, you can contact the Samaritans on 116 123 via phone for a confidential chat or email jo@samaritans.org.
