Completed during 1846 and opened on 30 June that year, the tunnel is approximately 1,300 yards (1,200 m) long and cut through the prominent Thackley Hill to reduce travel time. The contractor chosen to build the Tunnel was James Bray, an iron and brass founder from Leeds who later contracted the construction of the Bramhope Tunnel on the Leeds to Thirsk main line.
As originally built, there was just a single tunnel bore at Thackley that contained a pair of lines. However, during 1900, the railway was increased to four tracks, the additional two lines being contained within a newly-constructed second tunnel bore. After this point, one bore was used to carry the Fast lines from Leeds while the other was occupied by the Slow lines. During 1968, the original Southern tunnel was closed, coinciding with the final closure of the Great Northern Branch Line from Shipley to Laisterdyke via Idle and Thackley. As of 2018, the later-built Northern tunnel remains in active use, while the disused Southern tunnel is subject to periodic maintenance activities.
As early as 1830, the first proposals to construct a railway to connect the towns of Leeds and Bradford appeared before Parliament; according to author Graeme Bickerdike, these ambitions had been stimulated by the burgeoning wool trade then present in Bradford. The initial 9.5 mile-long direct route promoted was viewed as posing some difficulties, particularly in terms of steep gradients in placed; at one location towards the western section of the line, it was envisioned that a stationary steam engine would have had to have been used to assist trains in the ascent of a 1:30 incline. As a result of inflating cost estimate, backers of the project were scared into withdrawing, leading to the failure of the first Parliamentary Bill. Nine years later, in 1839, a revised scheme for such a route was put forward, but this failed to secure enough external financing and thus was shelved again.
Another four years passed before the project gained the attention of the famed railway financier and politician George Hudson, who later became known as ‘The Railway King’. Bickerdike credits Hudson's intervention as having playing a critical role, when the project again experienced difficulty in raising sufficient capital, Hudson himself offered a guaranteed return of 71⁄2 per cent to prospective investors, leading to a surge in demand to be involved in the initiative. The project also benefitted from the expertise of engineer and famed railway pioneer Robert Stephenson, who was consulted from an early stage. Stephenson surveyed a new route for the proposed line along the Aire Valley, thereby entering Bradford from the north.
Perhaps the most significant feature of Stephenson's proposed route was a 1,364-yard tunnel that would have to be cut through Thackley Hill, which otherwise obstructed the line. While some figures opposed the new route on the grounds of it being around four miles longer than the original plan, Stephenson stood by and defended his decision. He explained to the reviewing Parliamentary committee that the revised proposed line's ruling gradient of 1:200 would be more suited to the relatively low-powered locomotives available during the era, which in turn meant that end-to-end journeys would actually be quicker and cheaper to operate than it would have been if traversing the more direct route.
Having been sufficiently convinced, during July 1844, the project received Royal Assent, clearing the way for construction to proceed. Accordingly, the staking out of the line got underway immediately, including at Thackley Hill, where seven shafts were started under the direction of resident engineer Francis Mortimer Young. In the following months, a condensing steam engine, generating up to 25HP, was installed for lowering both men and equipment into these shafts, which eventually reached depths of as much as 252 feet, as well as for raising spoil from them. During January 1845, a £68,000 contract for the substantive works involved in the tunnel's construction was awarded to Messrs Nowell & Hattersley.
Conditions experienced by the tunnel workers were extremely challenging and worsened by the rudimentary working practices and lack of safety measures. Construction activity was near-continuous, working around-the-clock in eight-hour shifts for six days a week, breaking only on Sundays. Accidents were commonplace and deaths were also a frequent occurrence, although it was commonplace at the time to attribute these incidents as having been the result of personal error. Their efforts were personally praised by Hudson, who spoke of their energy and spirit during a celebratory meal marking the tunnel's completion after sixteen months of work. On 30 June 1846, the route, commonly known as the Airedale Line was formally opened to great fanfare and public spectacle, while a special train departing from Leeds carrying shareholders and other key figures ran in the early afternoon.
Upon its completion, Thackley Tunnel actually had a length of 1,496 yards, a full 132 yards longer than had been initially planned. Five of the working shafts were retained and reused as ventilation shafts. Operations using the tunnel were viable but not trouble-free; at least three people have been recorded as having died while in the completed tunnel during the 1800s, while a number of derailments and floods also sporadically occurred. However, the tunnel proved to be a commercial success and was in ample demand throughout this period.
During July 1897, as a result of increasingly high levels of traffic using the route, it was determined by the Midland Railway’s Works Committee that a second bore would be required to supplement the original. During 1898, Royal Assent was granted to the ‘Thackley Widening Act’, allowing for contractor Thomas Oliver & Sons and engineer J. A. McDonald, to commence work. On 27 January 1901, the second tunnel bore was officially opened to traffic; this allowed for the line to be increased to four tracks, the additional two lines running within the newly-constructed second bore.
During 1968, during the Beeching cuts, the original bore was rationalised[disambiguation needed] and traffic was reallocated to use the second bore alone. As such, the first bore has been left in a state of disuse; as of 2017, its structure has continued to be maintained and is subject to infrastructure manager Network Rail’s asset management regime, akin to its live counterpart. In April 1985, a bulge was recorded at the haunch closest to the live bore; initial steps to address this included the installation of several steel ribs to brace the afflicted area and the adoption of additional monitoring measures. During the 1980s, the closed original tunnel was gradually backfilled with spoil as a preventative measure against further distortion of the tunnel's lining.
During 1992, a pair of blockwalls were built in the first bore, the 83-yard section of tunnel between them was grouted, along with No.3 shaft, to prevent further structural deterioration. This work limited any potential for reusing the tunnel as it was done without any provision for through-access, effectively cutting the first bore into two separate halves. During 2013, inspections recorded the appearance of further distortions in the roof of the first bore that generated concerns that consequential defects may soon emerge in the operational bore which would be difficult to remediate due to the presence of overhead line equipment. In response to this development, further infilling activities, principally using lightweight foam concrete, were performed over a distance of 67 yards during mid-2016; these modifications have been designed to contain the likelihood of an eventual failing of the tunnel lining.
- In 2003, Russell Trueman, councillor for Thackley, decided to erect a fence around the surrounding area of the tunnel to prevent people from climbing onto the train tracks.
- In 2010, Thackley resident Harry Hurrell petitioned for the tunnel's closure on the grounds of unspecified safety reasons.