Around 70 passengers evacuated and line closed after Southeastern train hits cows that wandered on to track between Ashford and Canterbury.
Southeastern Trains said the incident happened between the villages of Wye and Chilham in Kent, involving a train travelling from Ashford to Canterbury.
Sixty-five passengers were helped to safety and Network Rail has shut the line. A spokesman said it was thought five cows had been killed.
Network Rail said it had received a report of a stray cow on the line about an hour before the derailment.
The cows also knocked out the radio and so the driver jumped out of his cab and ran down the line to stop an approaching train.
He then used the radio on that train to alert the signaller and turn off the power to the line.
The railway line will remain out of operation for two days while the investigation is completed, according to Network Rail.
