Train services between Huddersfield and Leeds via Brighouse are being diverted after a person was found dead on the tracks.
National Rail said disruption is expected until 10am.
The line between Halifax and Huddersfield is completely blocked, train operator Northern said.
A British Transport Police said: "Shortly before 4.10am today (16 April) British Transport Police were called to the tracks close to Brighouse station following reports of a casualty on the tracks.
"Paramedics also attended however a person was sadly pronounced dead at the scene.
"Officers are now working to identify the person and inform their family, and to establish the circumstances that led to the incident."
Train services from Leeds to Manchester Victoria / Southport and Wigan Wallgate will be diverted via Bradford Interchange and will not be able to call at Cottingley, Morley, Batley, Dewsbury, Ravensthorpe, Mirfield and Brighouse.
Rail replacement transport is available at Halifax and Huddersfield.
Services from Leeds to Manchester Victoria via Bradford Interchange and TransPennine services from Leeds to Manchester Piccadilly via Huddersfield are not affected by this problem.