Trains were delayed between Sussex and London after a dog owner tried to climb on to a railway track to rescue his pet.
A spokesman for Sussex Police said they were called to Haywards Heath station after a man "indicated strongly" that he would climb down to retrieve the dog.
A Southeastern spokeswoman said they believed the animal had been hit by a train at the entrance to a tunnel.
The man was escorted from the railway station by officers.
People travelling between London and Haywards Heath, Eastbourne, Littlehampton, Brighton and Ore on Monday evening were affected.
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