The Potters Bar Tunnel is a railway tunnel on the East Coast Main Line between Hadley Wood and Potters Bar railway stations; at in length, it is the longest on the line. Its two parallel bores carry four tracks beneath high terrain and the M25 motorway. Each bore is in length and tolerate speeds of on the slow tracks and and on the fast tracks.
The first bore of the tunnel was designed by Thomas Brassey and finished for the opening of the line in August 1850. Despite permission being granted by Parliament in 1882, a second bore was not opened until 1959 due to the associated cost. It is because of this that Potters Bar did not see the same expansion as towns to the south served by the line.
The Potters Bar Tunnel consists of two parallel bores, which carry the four tracks of the East Coast Main Line (ECML); each bore contains two tracks for one direction of travel, towards (Up) or away (Down) from London King's Cross. Both of the tunnel bores are in length, and are situated to from the zero point at London King's Cross railway station, from where mileage on the ECML is measured. The tunnel lies at the northern end of the longest section of the ECML with a constant gradient, with the line having risen at a gradient for by the northern portal. This makes the northern end of the tunnel the highest point on the line between London and Newcastle upon Tyne.
The speed limit through the tunnel is on the two slow tracks, on the Up Fast track, and on the Down Fast track. The speed limit then drops to and on the Down Slow and Fast tracks, respectively, at the southern portal of the tunnel. The northern end of the tunnel lies at the start of Potters Bar, meaning that the railway line then cuts directly through the town and restricts eastâÂÂwest travel. However, a footpath does pass over the tunnel mouth.
The British engineer Thomas Brassey was in charge of the construction of the Great Northern Railway between Hornsey and Peterborough. The construction of the tunnel and line through Potters Bar was long before the town was chosen to have a station, and Brassey set up his own brickworks and shops beside the line in order to avoid overcrowding the town; this also involved illegally paying the workers in tokens only they could use at the shops. The line through the station and the tunnel were completed by February 1850.
The original railway was entirely two tracks but was soon quadrupled to increase capacity. While an Act of Parliament passed in 1882 permitted this work to end at the northern end of the Potters Bar Tunnel, none of the three tunnels between Hadley Wood and Potters Bar were actually quadrupled due to the high costs of tunnelling second bores. It was due to this that Potters Bar was not able to have as frequent a service as the stations further south, preventing its expansion. A plan existed to quadruple the section at latest by 1892, but the decision was taken to extent the Hertford Loop line to Stevenage as an alternative route instead.
The construction of the second bore had been permitted by the original Act of Parliament, so it was carried out based on the permission sought 77 years beforehand. The works were the first instance of clay tunnel boring on the UK railways since works on the Hertford Loop line in 1914, 45 years earlier. The project was approved by the British Transport Commission on 30 April 1953, with construction beginning on 16 June 1955.
The works were carried out while the adjacent extant tracks were still live with no barrier in between; however, a temporary speed limit in the area of was created. The main problems faced by the contractors were that the tunnel had to be installed while other parts were still being excavated due to the weakness of the London Clay and that the deposited spoil was unstable and shifted over time. They overcame the latter problem by compressing the soil between excavating and dumping it. The tunnel was officially opened on 20 May 1959, and the success of its construction techniques were applied by the London Transport Executive to their plans for the Victoria line. At the time they were building experimental tunnel sections with similar techniques, while they awaited final approval for the entire line's construction.
The first section of what is now the M25 motorway, then known as the M16, opened between South Mimms and Potters Bar in September 1975; the Potters Bar Tunnel runs beneath this section. , the operators passing through the Potters Bar Tunnel include Grand Central, Great Northern and Thameslink, Hull Trains, London North Eastern Railway, and Lumo.