A passenger has died on board an Avanti West Coast train travelling between London and Liverpool.
Officers were called to Watford Junction station, Hertfordshire, just after 15:00 GMT following reports of a casualty.
Avanti confirmed the train had to stop and passengers were asked to disembark.
British Transport Police (BTP) said "a person has been pronounced dead at the scene" and the incident was not being treated as suspicious.
The train was believed to be the 14:43 from London Euston to Liverpool Lime Street, which has scheduled stops at Milton Keynes Central, Crewe and Runcorn.
BTP said: "Officers were called to Watford Junction station at 15:02 following reports of a casualty on board a train.
"Paramedics also attended however sadly a person has been pronounced dead at the scene.
"The incident is not being treated as suspicious and a file will be prepared for the coroner."