The north portal of the tunnel featured in
SO6510 : South portal of Blue Rock tunnel. The caption for that picture will also do for this one, so ...
Blue Rock tunnel was built to carry the Forest of Dean Railway through a rocky outcrop between Ruspidge and Soudley; it was the northernmost of three tunnels in quick succession on the route between Bullo and Cinderford.
As with many of the Forest of Dean lines, the history of the railway is, shall we say, involved, marked as it is by tramways, insolvencies, ownership changes, more insolvencies and lord knows what else, but loosely speaking, the Forest of Dean Railway evolved from the failing Bullo Pill Railway which opened as a private four foot gauge tramway in 1810. The FoDR (as distinct from the Forest of Dean Central Railway, a quite different company) rebuilt much of the line, including the building of two tunnels including Blue Rock: Haie Hill tunnel, which had been constructed by the tramway company (and at the time was the longest tunnel in the world at nearly 1100 yards) was enlarged to allow railway use. The line closed in 1967, traffic having dwindled over the preceding few years.