British Transport Police officers were called to the line close to Wivelsfield rail station in the Mid Sussex area.
The call came into police at 10.24am.
Officers attended alongside paramedics, however a person was pronounced dead at the scene.
British Transport Police say officers will now look to identify the person and inform their family.
The incident is not being treated as suspicious. A file will be prepared for the coroner.
The line has been shut for most of the morning with major delays and cancellations to services on the line to Brighton.