I am all for re-opening old lines, but in this case I think it would be nigh impossible.
Bangor (from Menai Bridge) to Caernarvon would be a good choice for a rail re-opening as Carnarvon is a very busy town, with a lot of tourist business, and further down the Llyn peninsula is a popular seaside area, although not too busy, probably because of the lack of railway infrastructure in the past.
However, just south of Menai Bridge the trackbed has been built upon by roads, and at Caernarvon it has been used by the Morrison's supermarket, so if you build a line to Caernarvon it would have to be mostly on a new alignment.
Through Carnarvon would be practically impossible, all the railway infrastructure through the town has been totally re-built until you get to where the current Welsh Highland Railway line is using the trackbed at the south end of the town.
From Caernarvon down to Afon Wen the trackbed is still in situ and used by the aforementioned WHR, as far as Dinas Jct, after this the trackbed is still in place although a few properties have encroached upon it, so it may be possible to re-open this stretch of the former Nantlle Railway/Caernarvonshire Railway, running alongside the WHR for a while, but the population is not high and business, tourist or otherwise, is little, so the only demand for this part of the line would be to link up with the Welsh coast line as a through route, but is there enough demand from that line for such a project?
Such a line would have to use a new alignment for stretches from Menai Bridge to Caernarvon and have to go around Caernarvon instead of through it, losing much of the traffic that a station in the town would generate, so in this case I think it would be a very nice pipe dream, Unfortunately
