To reach its junction with the South Wales Main Line, the Wye Valley Railway first had to penetrate a limestone outcrop beneath Wallhope Grove. This was achieved by means of the 1,188-yard Tidenham Tunnel which took trains from the steep-sided gorge at its northern end into green and pleasant farmland.

Constructed by Reed Bros of London, it consumed the lion's share of the route's £318,000 overall cost, taking two years to build at a rate of six feet per day and extending much further than its planned length of 715 yards. 

Today the tracks are still down but. unsurprisingly, rusted up. A couple of minor rock falls have occurred. Though generally dry, the tunnel features some remarkable localised mineral displays.August 2008 saw British Railways Board (Residuary), Tidenham's owners, erected palisade fences across both entrances. But access remains possible for the four species of bat found hibernating within it.

Official closure came in 1986.