A 40-year-old man from Littlehampton suffered a cardiac arrest after his dog pulled him onto the train track at West Worthing railway station.
A British Transport Police (BTP) spokeswoman confirmed the man was taken by ambulance to the Royal Sussex County Hospital in Brighton.
He is thought to have been in a mobility scooter.
The man’s condition is currently unknown. Police said his dog was uninjured.
A BTP spokeswoman said: “We got called at 3.56pm to say a man had been pulled onto the tracks at West Worthing station by his dog.
“He went into cardiac arrest as a result of the fall. He was taken to the Royal Sussex Hospital by ambulance.
“The man involved is a 40-year-old from Littlehampton in West Sussex.
“The dog is fine.”
Rail services were severely disrupted between Brighton and Portsmouth with a number of delays and cancellations.
The line was reopened though delays continued into the evening.
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