I've not run across any. Friends that have been there made a few suggestions plus a thorough study of Google Maps helped me plan out my trip. Go to the festrail website, click on Timetable and then click on the various station names in blue. This will bring up a segment of the UK Ordinance Survey Maps (equivalent to our USGS) maps showing the area around the stations. You can move these maps beyond the area initially shown. I didn't, but you may want to subscribe to the Ordinance map service so you can have real time access on your phone or tablet.
Locations I would suggest are obviously Porthmadog station and along the Cob (the long seawall that the tracks run on top of), Tan-y-Bwlch station, Ddualt station and vicinity (only accessible by train) and the area around Tanygrisiau and Blaenau Ffestiniog. I spent much of my time hiking around Ddualt so cannot give any good advice about these last two locations. If you do go hiking, do not walk along the ROW. Clearances are nil with stone walls lining the ROW for much of the way. Use the public foot paths or simply hike overland in the national park.
Concur with advice to carry rain gear. I had sun one out of the three and one half days I spent there in April.
Roadway width on the main highway between Porthmadog and Blaenau Ffestiniog is not a problem. Where you will encounter narrow roads, some less then 2 lanes wide (when the center line vanishes, watch out!) with slate fences literally at the roadway edge is along the WHR between Pont Croesor and Rhyd Ddu. Parking is pretty much limited to station parking areas plus there is a lay by along the river near Aberglaslyn Pass.
For Garratts, shoot the train leaving Porthmadog via street running crossing the Victoria bridge. Then head for Aberglaslyn Pass and shoot across the river. Then head for Rhyd Ddu, where the trains cross. From here to Caernarfon is down grade so you may want to break off and head back to the Ffestiniog, shooting the train along the river again, this time chimney first. Then head for Pont Croesor and catch it again or head directly for Porthmadog and catch the arrival.
Good luck and enjoy your trip. It's a fantastically scenic area and the trains are polished to a high standard.
Michael Allen
Edited 1 time(s). Last edit at 08/26/2018 10:14AM by trainrider47.