Athy,
The Welsh Highland Railway from Portmadog to Caernarfon is part of the Festiniog Group now, and they connect at Wharf Station, which is currently being remodelled to provide a proper platform for the WHR. They have run trains from BF all the way to Caernarfon.
The Welsh Highland Heritage Railway was the group that first wanted to build the through line, but for various reasons weren't able to. There was a deep division between the two companies, but this seems to have mellowed a lot now, and the WHHR have a connection to the WHR at Gelerts Farm, and that has been used several times now. Unfortunately the two companies use different braking methods so they do not have compatibility, but it looks like a solution is being worked on.
Still trying to find the time to do the Port - Caernarfon trip!