I am guesing this is the old rail bridge from the mills to the waste tips that crosses the stream near the bottom of the incline from the Rhiwbach tramway.
http://www.bbc.co.uk/news/uk-wales-north-west-wales-20025117 A 16-year-old boy has been airlifted to hospital after falling 20ft (6m) onto rocks from a bridge at a Gwynedd slate quarry.The incident happened at Maenofferen, Blaenau Ffestiniog, on Sunday evening.
A rescue team helped place the boy in a special stretcher and he was winched into an RAF rescue helicopter.
The aircraft, from Valley on Anglesey, flew him to Ysbyty Gwynedd in Bangor with multiple injuries.
A rescuer said the boy fell on such a hard surface he was very lucky not to have been more seriously hurt.
Been injured while at work and are not to blame?
Get over it.