Rheilffordd Ffestiniog Railway
Rheilffordd Ffestiniog Railway is an amalgam of the Welsh and English names of the railway. The term is used in publicity at various times.
Rheilffordd Ffestiniog for the Welsh speaking visitors and
Ffestiniog Railway for the English speaking visitors.
(This is the author's personal thought, I don't know if there is any other background.)