Hi Bill,
You are correct, there is no loading gauge problem for the larger WHRwy equipment from the Boston Lodge works to the Ffestiniiog/Welsh Highland Station at Porthmadog. Just beyond Boston Lodge, there is a highway bridge that blocks WHRwy coaches and locomotives, although the Ffestiniog does have at least one of the large South African gondolas which they can use throughout the Ffestiniog system (I believe they use it for coal deliveries).
By the way, in an unrelated note, the Ffestiniog has run several STEAM lumber trains this year. There is an area alongside the Ffestiniog which is having lumber removed, and they used one of the Fairlies to haul the lumber train to the unloading location - quite an interesting series of photos!
Phil