1873 marked the opening of the Ross & Monmouth Railway which crossed the River Wye twice and penetrated two outcrops of rock with tunnels. The longer of these, at 630 yards, had its south portal at Welsh Bicknor but took the name Lydbrook. It was situated across the water from Lydbrook Junction Station where the Severn & Wye line branched off towards Lydney.

Both sides of the approach cutting benefit from lengthy retaining walls, resulting in a rather diminutive portal. Inside the tunnel is stoned-lined at both ends, giving way to a brick lining through its central section. Parts of the roof near the northern end have been repaired in red brick and bulging is apparent in places. Trains travelled on a northerly course through the southern half of the single-track bore before taking a tight curve to the west. Refuges of varying dimensions are provided throughout and cable hangers remain in situ on the west wall. Like its distant sibling, the north portal is also flanked by retaining walls. Maps suggest that there was an air shaft around 140 yards from the southern entrance but there is no sign of this within the tunnel.

Whilst the through route was closed in 1959, the northern section of line – including the tunnel – remained operational until 1st November 1965 to serve the AEI cable works siding at Lydbrook Junction. By the summer of 1967, the tracks were gone and the tunnel bricked up. Since then, large holes have formed in both walls providing easy access, although flooding affects the southern end.