Transport for Wales said a single line is blocked with disruption expected until 12pm.
It said journeys to and from these stations may be cancelled, delayed by up to 90 minutes or terminated and returned to Pontypridd.
A notice on the Transport for Wales website reads: “A person has been struck by a train along the Taff Valley route.
“All trains to Merthyr Tydfil will be terminated at either Pontypridd or Abercynon.
“Replacement road transport services are conveying passengers between Abercynon and Merthyr Tydfil via Merthyr Vale in both directions until further notice.”
Find out the latest traffic and travel news where you live:
A bus shuttle service will operate between Abercynon and Merthyr Tydfil calling at all stations, and the buses will follow the train timetable as closely as possible.