It's a triple marathon covering the 80 mile round trip starting from Porthmadog at 08:45. Breakfast is served on the train en route to Caernarfon, with lunch at The Goat in Beddgelert on the return to Harbour Station. Then it's up and down the Vale of Ffestiniog and back to the starting point at 18:15 if all goes to plan.
The very few first class tickets cost £400 but if you don't mind slumming it in 3rd class it's a mere £180. Booking details are here.
The last time I watched it going past it was running a bit late as it rounded Tank Curve and past Campbell's Platform.