Built between December 1838 and June 1841 for the Great Western Railway (GWR) under the direction of Isambard Kingdom Brunel, the straight tunnel descends on a 1 in 100 gradient from its eastern end.

[5] The Great Western Main Line would maintain either level ground or gentle gradients of no greater than 1 in 1000 along most of its route.

[7] The GWR selected George Burge of Herne Bay as the major contractor,[7] being responsible for undertaking 75 per cent of overall tunnel length, working from the western end.

[7] By summer 1840, the London Paddington to Faringdon Road section of the Great Western Main Line (GWML) had been completed, as was the track from Bath to Bristol Temple Meads.

A special train departed London Paddington and traversed the whole of the GWR to complete the first rail journey to Temple Meads Station in Bristol in about four hours.

The height of the opening is far in excess of what was required (and indeed reduces once inside), but it gives the feel of a generous celebratory monument to a new form of travel.

[7] Commentators and critics voiced concerns and disapproval about the unlined section of the tunnel; they believed that it lacked solidity and was a danger to traffic.

[5] The GWR responded to these complaints by building a brick arch underneath part of the unlined section close to the entrance which was prone to frost damage.

In the run-up towards the Second World War, the need to provide secure storage for munitions at distributed locations across the UK was recognised.

During the 1930s, a proposal to create three Central Ammunition Depots (CADs) was submitted: one in the north (Longtown, Cumbria); one in the Midlands (Nesscliffe, Shropshire); and one in the South of England at Tunnel quarry, Monkton Farleigh and Eastlays Ridge.

During November 1937, the GWR was contracted to build a 1,000-foot-long (300 m) raised twin-loading platform at Shockerwick for Monkton Farleigh and two sidings branching from the Bristol–London mainline just outside the tunnel's eastern entrance at 51°24′19.31″N 2°17′22.94″W / 51.4053639°N 2.2897056°W / 51.4053639; -2.2897056.

[13] BAC used the facility to accommodate the company's experimental department, which was developing an engine to power bombers and the Bristol Beaufighter.

During the post-war years, portions of the ammunition depot were redeveloped for other facilities, including the Central Government War Headquarters, RAF No.1 Signal Unit, Controller Defence Communication Network and the Corsham Computer Centre.

During the summer of 2015, the tunnel was closed for six weeks for preparatory work including lowering the track by roughly 600 millimetres (24 in) and replacing 7 miles (11 km) of cabling in advance of the catenary infrastructure being installed.

[4][15] However a November 2016 announcement stated that the plan to electrify the mainline from Thingley Junction (near Chippenham) to Bristol Temple Meads was delayed indefinitely.