Russ,
A day worth being there for is the Snowdonian special. It is April 18, 2020 and runs over the entire combined WHR and FR systems. Since it is in addition to the regular service trains and tends to be double headed, they have to run just about every loco they have to accommodate the special and the service trains. Two years ago I saw double headed Fairlies on the WHR (happens very seldom) since both Garratts were on the special. It is a real chasers treat. Or you could book a ticket and ride in style, but it tends to sell out well in advance. If you do ride, try and book one of either the FR or the WHR's stunning first class observation cars.
Two Fairlies at Aberglaslyn Pass on the morning Porthmadog to Caernarfon service train.
Afternoon Caernarfon to Porthmadog service train with two Fairlies.
That is Mount Snowden behind the the double headed Garratt's pulling the Snowdonian through the big curve below Rhyd Ddu on its return to Porthmadog.
As for the weather, two years ago it was 2/3 rainy but the sun shown on the Snowdonian!
Michael Allen