Great pics in that clip ... when I saw the viaduct pic that was the original part of this quiz I remembered seeing such a construction from the Trawsfynydd-Bala road ... so did the usual Google Maps and came up with the first pic below. The others are the work of Dan Bovington who must have gone there with a drone, linked from google maps, so all credit to him.
Amazing constructions back then when you consider there was no heavy machinery, it was all hard labour. I also suspect no red tape back then, to create all sorts of delays! Major, and expensive, task to get these lines open again but what a tourist attraction they would become. This particular line to Bala was partly lost into Tryweryn Reservoir.