A few others no one has mentioned: Relatively close to the ffestiniog are the Welsh Highland, which is currently rebuilding the historic 21 mile line of the same name between Dinas Jct. and Porthmadoc. The Caernarvon end runs from within sight of the famous Caernarvon Castle and runs 3 miles to Dinas. The 3 mile section between Dinas and Waenfawr is expected to open in July. This line runs 2-6-2+2-6-2 2' gauge Garratts! The southern end at Portmadoc, just a few blocks from the ffestiniog's Harbor station, has some interesting sights too. The two lines are building towards one another through Beddgelert and Rhyd-ddu, by way of Aberglaslyn Pass. They will eventually connect with the ffestiniog in Portmadoc. There is also a great, mostly steam powered, railway in the same general vicinity that runs to the top of Mt. Snowdon. On the other side of the lake is the 2' Padarn Rwy.
South from Portmadoc, along the s.g. line through Barmouth but before you get to the Vale of Rheidol is Towyn, home of the famous Talyllyn Rwy. This one is a must see! On the way, the s.g. runs through Fairborne, home of the miniature Fairborne Light Railway. The Welshpool & Llanfair is somewhat removed from the others, but interesting. It is northeast of the Rheidol's terminus in Aberystwyth.
This sort of makes the point of many contributing to the marketing string. There can be prosperity in numbers.
One last line, removed from the others in south Wales, is the Brecon Mountain. They have a Baldwin 4-6-2 that has been beautifully rebuilt to resemble one of the large SR&RL, 2'gauge 2-6-2s.
Wish I could go with you.
BestRegards, Glenn