Tidenham Tunnel was constructed to carry the Wye Valley Railway through a mass of limestone rock, allowing the railway to pass out of the Wye Valley before it enters a deep, tightly-curved gorge on the approaches to Chepstow. The line emerges into arable country and can head down to the mainline on a much straighter and cheaper route than would be possible if the tunnel had not been bored and the railway had followed the Wye down to Chepstow instead. One ventilation shaft was provided, slightly to the north of halfway through the tunnel. A distinct curve at each portal ensures that the bulk of the bore is in pitch blackness as it passes through the hill. Much of it is unlined, as the limestone was deemed to be solid enough to not require the additional expense of lining.

Following completion of the tunnel it opened to traffic with the line in 1876. For such a long tunnel on a railway with such a traumatic history it had a comparatively quiet time. The WVR was saved the expense and inconvenience of constant rock falls which disrupted the operations of other railways and the largely level and dry bore did not cause trouble because trains stalled inside or it flooded. By its very nature as a single track tunnel it was a confined space and unpleasant to work through with a steam-powered train but it didn't pick up the sort of reputation which its slightly shorter neighbour, Haie Hill Tunnel (about 30 miles further up the Severn) managed to obtain.

British Railways were left with the tunnel after nationalisation. The line cost a great deal to maintain and BR was keen to lose it in order to save money. The railway closed to passengers in 1959 but Tidenham Tunnel still had a fairly secure future with limestone traffic from Tintern Quarry. This could probably have gone on forever if it were not for the deteriorating state of the tunnel. The last serious attempt at maintaining it was probably carried out prior to 1959 (it is entirely possible that an overhaul of the tunnel was due when the line closed). Closing it to passengers had allowed less money to be spent on it; in December 1981 the maintenance requirement was reduced to virtually nothing by taking the line out of use. By this stage it was the 13th-longest active tunnel on the former Great Western network; this status now passed to Ardley on the line between Banbury, Bicester, High Wycombe and London.

The empty, two-thirds of a mile long hole now lies silent under Dennel Hill. Brambles, tree growth and rubbish made both portals increasingly inaccessible over the following 27 years, although neither was fenced off until August 2008. This necessitated clearing the portals; that at the south end used to be virtually inaccessible from Netherhope Lane. The old track remains in remarkably good condition - spared the pains of the Great Outdoors, the steel rails lead into the tunnel with only light oxidisation and the creosote in which they were intensively soaked seems to have helped the sleepers survive the following 50-odd years extreamly well. The tunnel is now inhabited by a population of bats.

The basic fact that there is track through the tunnel means that it is clearly a railway tunnel and much of the work which would need to be carried out in it could be carried out by rail-based vehicles. For example, one of Network Rail's test trains could be borrowed and taken through to get an in-depth analysis of the inside of the tunnel.

Thus re-opening would require a series of steps:

1. Enter the tunnel with a group of engineers and surveyors. Carry out an examination of the bore to ensure that it is structurally sound and could be restored to a suitable condition for the restoration of rail services. If it has started falling in then re-opening is not an option and it would be preferable to liquidate the owning group so the Crown can deal with it.
2. If the tunnel is in moderately good condition then it would be necessary to carry out bat surveys and work out mitigation procedures. Once these are dealt with a handy group of official bat lovers will be brought round to the tunnel, shown it in detail and informed of the various benefits of the plans and the effects of mitigation in the hope that they will provide some advice, even if they do not wholly support it. Hopefully this should result in a first-time acceptance of the scheme by the bat inspection groups. Work will mostly be carried out by a group of bat-approved contractors who will be treated as employees for the duration of the bat removal work. This will take place during a period when bats are not in residence.
3. Once the bats are removed the tunnel will undergo a thorough overhaul. Unlined sections may have to be shotcreted (covered in concrete fired out of a gun-like tube) to ensure that they remain secure. Refuges will be provided in the new walls.
4. The rest of the line will then be overhauled before work returns to the line south of Tintern Quarry. The old track will be used for construction trains until work is completed or it disintegrates; this would allow new track to be laid around Tidenham and Netherhope at the same time as the stations are overhauled and rebuilt for passengers, thus giving a nice new air to proceedings. It would also reduce the work to be carried out while the bats are absent.
5. If the bats are happy to return to an active tunnel (and they probably will be, given some recent experiences by a building restoration group in west Monmouthshire, whose bats were distinctly unimpressed by their alternative home) then they can continue to hibernate in the tunnel.

Previously we have talked down the tunnel as a fairly minor affair, but it has risen up the priorities list to be the greatest obstacle. It includes a great deal of public relations work concerning the noise and the effects on the bat population; the bats and noise also provide environmental issues, while rebuilding such a long structure is also a major engineering challenge. While the engineering challenges around the Tintern bridge and the viaducts at Monmouth and Penallt can be carried out in the open air, providing lots of space for heavy lifting equipment, track will have to be removed from and placed in Tidenham Tunnel in an enclosed space and all engineering equipment will have to fit into this bore with suitable evacuation procedures (which can make only limited use of the ventilation shaft). This results in a very difficult task with few precedents. Single-line tunnels are now quite rare, the branch lines which they were on having mostly closed, and very few have been subjected to full overhauls in recent years.

Natural England kindly provides information on the procedure for dealing with applications to interfere with bats; as the tunnel is in England this is a useful guide. It is mostly common sense (a phrase often used by Planning Department staff dealing with instructions mere minutes before they misconstrue them).

Basically, the work must be in the overriding public interest (well, we think it is) with no other satisfactory alternative (bore another tunnel?) and a favourable conservation status (which sounds like somewhere else must be found for the bats). Proposals should be firm and confident with minimal dithering; only relevant information should be provided with no waffling; you should be precise about the number of bats being dealt with. They like to feel that you know what you are doing with their bats. There is also the query as to whether a licence is actually needed to deal with the bats; that will probably depend on whether it is decided that the bats need to go or not. The proposal should be clearly, although not too precisely, timetabled.

One thing which we can claim is that people will only enter the tunnel on foot for maintenance, which we will try to schedule for out-of-season periods. Otherwise the bats will be left to enjoy life, except for the occasional passing train. If they hide in the recesses in the walls then they probably won't notice. When trains are running people will be discouraged from entering the tunnel to disturb the bats by the proximity of an active station to the southern portal, the difficulties of accessing the northern portal and the inherent dangers in wandering around on railway lines. Provisions will be made for the tunnel to be securely gated overnight to keep out explorers, but the gates employed (which will be similar to, but slightly higher than, those protecting the southern portal of Tintern Tunnel) will provide plenty of space for the bats to fly in and out.

The ventilation shaft will undergo a cursory examination but unless any notable problems come to light it should be possible to leave it undisturbed. It is unlikely to be used by bats due to it being illuminated during the day.