Originally a horse-drawn tramway, the Bullo Pill Railway carried coal, iron and stone from the Forest of Dean to the tiny port of Bullo Pill on the River Severn's west bank. Authorised by a Parliamentary Act in 1809, its 1,083-yard Haie Hill Tunnel, engineered by John Hodgkinson, was the world's longest when it opened a year later. The bore was driven through the lower strata of old red sandstone by contractor Robert Tipping.

The line was taken over by the South Wales Railway in 1851 and broad gauge track was installed through the tunnel in 1854. It had to be enlarged to accommodate this, work which fell to I K Brunel who described it as "The most difficult task I have yet undertaken" as traffic continued to pass through as the work progressed. It also became shorter, losing 19 yards at its east end. Standard gauge took over in 1872. With passenger trains already gone, official closure of the line to all traffic came on 1st August 1967.

The eastern portal stands at the end of a stone-walled cutting, obscured by vegetation. It has been partly bricked-up although a low-level access hatch is provided. Inside, the bore is tidy and mostly dry, climbing a stiff rising gradient of 1:56 to the west. It could take a train of empty wagons five minutes to pass through. And the children of Bullo Pill used the tunnel to reach their school at Soudley, timing their walks so as not to meet any rail traffic. Although mostly straight, a slight curve to the north is encountered at the west end.

The masonry lining features an arched roof with vertical side walls into which generous refuges are provided at regular intervals, some with exposed rock at their rear. Signalling pulleys and cable hangers remain in situ on the south wall. Near its centre, a rare milepost remains.

A drain runs down the tunnel's centre line - accessed via numerous small catchpits - whilst many weep holes have been cut into the lining, resulting in some significant accumulations of calcite on both walls. At the west portal, which is also bricked-up, a stream runs in a channel beneath the bore.