Are you sure the overbridge was built by BR? I thought it was built when the new road to TanyGrisiau power station was built (going over that overbridge).
Firstly, I'd like to apologise for not responding to your question earlier, but I've been otherwise engaged with family matters.
In my post, I questioned the Wikipedia date of opening of the connection as 1964, but I've subsequently found (in WG Rear's book about the Blaenau Ffestiniog branch) a reference to it being officially opened on 20th April, 1964; moreover, there are a number of photos reproduced in the book showing the connection, and all are also dated 1964, and one of them clearly shows the new bridge. However, the connection must have been completed for some considerable time before it was officially opened, as another photo - dated August 1963 - also shows it. The earliest self-taken photo I have showing the connection was taken in 1964.
The photo on the front cover of Bill Rear's book is of a pair of class 108 DMUs at Blaenau Ffestiniog North; the photo is taken looking towards the buffer stops, and is dated 1959. The
"new" bridge hasn't been constructed at that time.
This verifies my recollection of the area as it was in 1958/9; I was taken to see both ends of the Moelwyn Tunnel and walked most of the FR trackbed between Tan-y-Bwlch and Duffws in those summers, and I have an undated photo of the old Tanygrisiau station building on a later visit to the upper stretches of the FR which I guess was taken in 1961 (I'm basing that upon it being on the same film as a photo of the FR's 'Merddin Emrys' running cabless and in red primer). I THINK it was on this 1961 walk that I saw the narrow gauge slate train entering the exchange sidings along the link from the Oakley Quarry mentioned in my original post.
I have photos taken on the FR in summer 1962; I can date those with accuracy because one is of 'Linda' undergoing tests following its arrival from the Penryn Quarry. So based upon what I can verify from elsewhere, it must have been this summer that I saw the connection/bridge under construction at Blaenau Ffestiniog (although some trips to the FR were the
"long way round" via Caernarfon and Afon Wen, most were via Blaenau, hence my recollections of the locality).
So there must still have been some slate moved by rail from Blaenau Ffestiniog in 1962, and it must have been assumed that it would continue, as the bridge works were undertaken to facilitate its continuation.
So that brings me to the road to the power station.
To be honest, I can't remember it ever NOT having been there, and I'm as sure as I can be that it was there when I took the 1961 walk to Tanygrisiau; moreover, I THOUGHT it was there when I was taken along the FR between North station and Duffws in either 1958 or 1959
I don't know who funded the overbridge which carried the road over the new connection - Liverpool Corporation maybe? - but it was built by BR independently of the road to the power station; I'm as certain as I can be of that, and also that it pre-dates the power station road. However, I haven't been able to find any references as to when the power station road was built, but I do remember going along it once on the connecting Crossville bus which was run some years in the early 1960s between BR at Blaenau Ffestiniog North and the FR at Tan-y-Bwlch. Again, that doesn't really help with dates or whether or not the power station road pre-dated the overbridge!
I'm conscious that this has gone a bit off-topic, but I thought the question deserved a reply, and I've given as much information as I can regarding dates based upon my own photos and dated ones in Bill Rear's book.
But to bring it back onto the topic of slate from North Wales, I can't recall ever seeing any narrow gauge slate wagons in the ex GWR station yard at Blaenau; I do recall seeing a freight on the branch from Bala Junction, but I can't remember what it consisted of so it may have contained some slate. I guess that would also have been 1958 or 1959.