Potters Bar Tunnel
| Potters Bar Tunnel | |
|---|---|
A train in the cutting between the Potters Bar Tunnel (L) and the Hadley Wood North Tunnel (R). The tunnel mouth of the former can be faintly made out on the very left. | |
Interactive map of Potters Bar Tunnel | |
| Overview | |
| Line | East Coast Main Line |
| Location | Potters Bar, Hertfordshire |
| Coordinates | 51°40′59″N 0°10′48″W / 51.683°N 0.180°W |
| Status | Operational |
| System | National Rail |
| Operation | |
| Constructed | First bore: 1850 Second bore: 1955–1959 |
| Owner | Network Rail |
| Operator | See East Coast Main Line § Operators |
| Technical | |
| Design engineer | Thomas Brassey |
| Length | 528 yd (483 m) |
| No. of tracks | 4 |
| Track gauge | 4 ft 8+1⁄2 in (1,435 mm) standard gauge |
| Electrified | 25 kV 50 Hz AC |
| Operating speed | Fast tracks: Down, 115 mph (185 km/h); Up, 105 mph (169 km/h). Slow tracks: 75 mph (121 km/h) |
The Potters Bar Tunnel is a railway tunnel on the East Coast Main Line between Hadley Wood and Potters Bar railway stations; at 1,214 yards (1,110 m) 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 528 yards (483 m) in length and tolerate speeds of 75 mph (121 km/h) on the slow tracks and 105 mph (169 km/h) and 115 mph (185 km/h) 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.
Design
The Potters Bar Tunnel consist of two parallel bores, which carries the four tracks of the East Coast Main Line (ECML); one bore carries the two up tracks (for trains travelling towards London King's Cross), and the other bore carries the two down tracks (trains travelling away from London King's Cross). Both of the tunnel bores are 1,214 yards (1.1 km) in length, and are situated 11 miles 25 chains (11.31 miles, 18.21 km) to 12 miles 00 chains (12.00 miles, 19.31 km) from the zero point at London King's Cross railway station, from where mileage on the ECML is measured.[1]
The tunnel lies at the peak of the longest section of constant gradient on the entire ECML, where the line rises at a 1 in 200 (5 ‰) gradient for 8 miles (13 km).[2] This puts the northern end of the tunnel at the highest point on the line between London and Newcastle upon Tyne.[3]
The speed limit through the tunnel is 75 miles per hour (121 km/h) on the two slow tracks, 105 miles per hour (169 km/h) on the Up Fast track, and 115 miles per hour (185 km/h) on the Down Fast track. The speed limit then drops to 65 miles per hour (105 km/h) and 100 miles per hour (160 km/h) on the Down Slow and Fast tracks respectively at the southern portal of the tunnel.[1] 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 east–west travel. However, a footpath does pass over the tunnel mouth.[4]
History
First bore
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.[5]
The original railway was entirely two tracks but was soon quadrupled to increase capacity.[5][6][7] While an Act of Parliament passed in 1882 permitted this work to end at the northern end of the Potters Bar Tunnel,[7] none of the three tunnels between Hadley Wood and Potters Bar were actually quadrupled due to the high costs of tunnelling second bores.[6][7] 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.[6] 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.[5]
Second bore
| External images | |
|---|---|
| Electrification work at the now double-bore tunnel, 1980s | |
| An LNER HST leaving the tunnel, c. 2018 |
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.[7]
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 15 miles per hour (24 km/h) was created.[8] 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.[9] The tunnel was officially opened on 20 May 1959,[7] 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.[9]
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.[3][10] As of March 2026, the operators passing through the Potters Bar Tunnel into King's Cross via the Gasworks Tunnel include Grand Central, Great Northern and Thameslink, Hull Trains, London North Eastern Railway, and Lumo.[11]
See also
References
- ^ a b "London North Eastern Sectional Appendix" (PDF). National Electronic Sectional Appendix. Network Rail. p. 206. 7 June 2025. Retrieved 6 December 2025.
- ^ Tuffrey, Peter; Brooksbank, B. W. L. (2022). "10: Gradients and Speed Restrictions on the ECML". The East Coast Main Line 1939–1959. Vol. 2. Fonthill Media.
- ^ a b The East Coast Diversion (PDF). Hertfordshire RailTours. 17 January 1998. p. 4.
- ^ "Potters Bar Urban Transport Plan" (PDF). Hertfordshire Highways. December 2011. p. 104. Archived (PDF) from the original on 29 June 2023. Retrieved 6 December 2025.
- ^ a b c "The Story of Potters Bar and South Mimms" (PDF). Potters Bar and District Historical Society. 1966. pp. 67–68. Archived (PDF) from the original on 12 August 2024. Retrieved 6 December 2025.
- ^ a b c "The Early History of Potters Bar" (PDF). Potters Bar Museum. March 2023. Archived (PDF) from the original on 18 July 2024. Retrieved 6 December 2025.
- ^ a b c d e Terris, Alexander Key; Morgan, Horace Denton (1 April 1961). "New Tunnels Near Potters Bar in the Eastern Region of British Railways". Proceedings of the Institute of Civil Engineers. 18 (4): 289–304 – via EmeraldInsight.
- ^ "Tunnelling 50 years ago". Tunnels and Tunnelling. 15 February 2007. Retrieved 14 March 2026.
- ^ a b Terrris, Alexander Key; et al. (1 April 1961). "Discussion – New Tunnels Near Potters Bar in the Eastern Region of British Railways". Proceedings of the Institute of Civil Engineers: 852–856, 862 – via EmeraldInsight.
- ^ "M25 – Dates". UK Motorway Archive. Archived from the original on 3 August 2020. Retrieved 14 March 2026.
- ^ "National Rail All Stations Route Map" (PDF). National Rail. 12 December 2025. Retrieved 14 March 2026.