Runaway Beddgelert locomotive is investigated
An out-of-control locomotive travelled more than a mile over a number of level crossings and through a station.
An investigation has been launched after the driver could not stop the locomotive on the Welsh Highland Railway near Beddgelert on 16 April.
It follows an accident in January where an engineering train on the Ffestiniog Railway ran into level crossing gates and stopped on a road.
The company that operates the lines said it is helping with the probe.
Ffestiniog and Welsh Highland Railways claims to be the oldest railway company in the world still operating trains.
It says the Welsh Highland Railway is the UK's longest heritage railway and runs for 25 miles from Caernarfon, past the foot of Snowdon, to Porthmadog.
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The April incident saw the locomotive heading down a steep hill, with the driver unable to stop it, a report by the UK government's Rail Accident Investigation Branch (RAIB) said.
It travelled at 10mph, through Beddgelert station, a number of level crossings and a danger sign.
The track levelled out after 1.25 miles and nobody was hurt.
The railway's general manager Paul Lewin was driving and he said: "It was awful - the brakes only seemed to be part working, but not slowing the locomotive down quickly enough.
"I knew there was no danger to passenger trains, as this was a light engine movement early in the day."
He said railway officials will work with RAIB investigators, while their own investigation is continuing.
An investigation "will establish the sequence of events and consider the factors that prevented the driver from stopping the locomotive, the maintenance of the locomotive (and) any underlying management factors," the RAIB said.
It follows an accident on the Ffestiniog railway in January, when an engineering train was travelling from Tan y Bwlch to Minffordd.
It ran into closed level crossing gates at Penrhyndeudraeth, pushing through them and came to stop on the crossing on the A4085 road.
The Ffestiniog Railway is described as the world's oldest narrow gauge railway with almost 200 years of history, running for 13.5 miles from Porthmadog to Blaenau Ffestiniog.
There were no injuries, but a RAIB investigation concluded with a reminder of the importance of following railway rules and organisations putting in place measures to make sure they are being followed.
"The consequences of a train running onto a level crossing into the path of a road vehicle can be fatal," it said.