A runaway train passed through a danger signal due to "an issue with the brake system", according to a report by the Department for Transport.
The driver was unable to stop the train as it travelled at 10mph down a steep hill through Beddgelert station.
No-one was hurt in the incident in Gwynedd on 16 April 2019.
An investigation report by the Rail Accident Investigation Branch (RAIB) said the "brakes had been modified".
The locomotive travelled through Beddgelert station, a number of level crossings and a danger sign.
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