The Gilfach Tunnel, located about 4 miles north of Rhyader is one of two tunnels on the Cambrian 'Spine of Wales' line. It is currently gated off and forms a bat rost. When I visited, a Tawny Owl was observed exiting the east portal. It is 1000feet long and straight. The southern portal is easily accessible, but the northern portal requires climbing through some sheep pens in full view of the road. It was abandoned in the 1960's