They were responding to reports a person had been struck by a train.
British Transport Police officers, who were leading the investigation, said the person was pronounced dead at the scene.
A spokesman said: “BTP officers were called to Green Lane, Haughley, after a report that a person has been struck by a train.
“Officers from BTP and other emergency services attended the incident, which was reported to BTP at 9.45pm and is currently being treated as non-suspicious.
“Medics from the East of England Ambulance Service attended and pronounced the person deceased at the scene.”
A file will now be prepared for the coroner.
All Greater Anglia trains have been stopped between the Bury St Edmunds and Stowmarket until the end of the day.
Services are being cancelled and delayed by up to 90 minutes or revised.