The incident is said to have happened in the early hours of this morning, Wednesday (May 30) in Kidwelly.
There were reports at approximately 5am that a person had been struck by a train. Emergency services responded and the man was pronounced dead at the scene.
Officers were called to the railway line near Kidwelly Viaduct and are currently investigating the circumstances surrounding the incident.
Disruption is expected until late Wednesday morning with certain lines being cancelled or delayed.
Disruption was expected until 11.30am.
Both lines have now been re-opened by British Transport Police however Arriva Trains Wales have said that commuters should expect delays while the service is recovered.