This is Porter's Lodge. It's the old porter's building at Glyndyfrdwy station. The main house next door is sometimes let too, but you'd have your own place.
It's right on the platform of the Llangollen Heritage Railway. You can sit out there with a drink and watch the steam trains go by. They're not constant, just a few services a day in season. It's actually very peaceful when they're not running.
The place sleeps two. One bedroom, one bathroom with a shower. It's got a 4 Star rating. It's comfortable and clean. It's railway themed, which really just means some old pictures and bits of memorabilia about the place.
There's a woodburning stove for cooler evenings. TV, WiFi, washing machine. Parking is right there. You can bring one well-behaved dog.
The village is quiet. There's a pub half a mile away that does food. The nearest shop is about three miles away in Corwen, so you'll want to bring your basics.
You're in the Dee Valley. Good walking straight from the door. Llangollen is a ten minute drive. You can walk the canal there or go over the Pontcysyllte Aqueduct. Chirk Castle and Erddig are a short drive away. Snowdonia is about half an hour's drive if you want bigger mountains.
Useful links: Llangollen Railway, National Trust Wales, and Snowdonia National Park.