The Welwyn North Tunnel and Welwyn South Tunnel are two railway tunnels on the East Coast Main Line between Welwyn North and Knebworth railway stations. They are and in length respectively, with an cutting between them. They each consist of a single bore, forming part of capacity problems on that section of the East Coast Main Line. They tolerate a speed of on the Down Main track; the speed on the Up Main track is in the North Tunnel and in the South Tunnel.
The tunnels were built in 1849âÂÂ50 as part of the Great Northern Railway between London and Peterborough. The southern portal of the South Tunnel is highly decorated and was given Grade II listed status in 2000 alongside the adjacent Welwyn North railway station. In 1886, there was a three-way crash in the North Tunnel caused by a broken-down train whose driver did not follow correct procedures; it would later be described as "one of the most destructive in railway history". In the modern day, the tunnels are frequently discussed as part of capacity issues on the East Coast Main Line which limit the trunk route to only 18 trains per hour.
Both tunnels consist of only a single bore, making them the closest tunnels to the terminus of the East Coast Main Line (ECML) at London King's Cross to do so. The nearest tunnels in either direction are the Potters Bar Tunnel, approximately to the south, and the Stoke Tunnel, approximately to the north. The South Tunnel is situated between and down the line from London King's Cross, from which distances on the ECML are measured, and the North Tunnel is situated between and .' The southern portal of the South Tunnel is situated around from the end of the platforms at Welwyn North railway station.
The speed limit through both of the tunnels is on the Down Main track; on the Up Main track, it is in the North Tunnel and in the South Tunnel. The speed limit on the Down Main track then increases to immediately after the northern portal of the North Tunnel, marking the most southerly point on the line where the line reaches its maximum speed; this means that Knebworth railway station is the first through which trains may travel at this speed.' The gradient of the tunnels and cutting between is .
The engineers who designed the layout of the Great Northern Railway were William and Joseph Cubitt. The tunnels were built between 1849 and 1850, and were the responsibility of Thomas Brassey, who constructed the line between Hornsey and Peterborough. Services on the line then began in 1850. The southern portal of the South tunnel, which is visible from Welwyn North railway station, was built from sandstone ashlar, as a semi-circular arch. The portal has towers on either side as well as a cornice and a parapet.
On 9 June 1886, there was a major rail crash in the North Tunnel, which L. T. C. Rolt described as "one of the most destructive in railway history". Initially, a fault with a Great Northern Railway locomotive led to it becoming stationary on the Down (northbound) track in the Welwyn North Tunnel. The guard wanted to move the train back to Welwyn railway station (now Welwyn North) by taking advantage of the falling gradient, but the driver refused as it was against the railway policy. The driver, however, did not take adequate steps to protect the train from a collision due to its being stationary. As the tunnel was full of smoke from the broken locomotive, the next train did not notice it and so hit it from behind. A third train, travelling southbound, then hit the debris from the collision which had covered both tracks; miscommunication between the signallers at Welwyn and Knebworth had meant neither were aware of the collision.
Capacity issues have been raised since as early as the 1890s, when the Hertford Loop line was extended to Stevenage in order to prevent having to widen the viaduct from two tracks to four. The project was completed in 1898, and the Hertford Loop line is still used as a diversionary route for trains today when there is disruption on the East Coast Main Line. The Department for Transport's 2006 study on railway capacity raised the issue again.
In August 2013, BBC News listed the idea of widening the viaduct and the adjacent Welwyn Tunnels to four tracks as one of the top five infrastructure projects that could improve life for people in the United Kingdom. It estimated the cost of such a project at ã440 million and considered its likelihood to be "probable, at some point". As of 2015 Network Rail set the maximum capacity of the bottleneck at 18 trains per hour (tph). The East Coast Main Line 2016 Capacity Review considered the quadrupling of the viaduct and Welwyn North railway station, concluding it would not fully resolve the capacity constraints", only increasing capacity for intercity trains by 2âÂÂ3tph.
On 4 October 2000, the southern portal of Welwyn South Tunnel was given Grade II listed status, which complements that of Welwyn North railway station's footbridge and station building, listed on the same date. On 24 October 2008, a broken rail in the Welwyn South Tunnel caused a northbound First Capital Connect service to become stranded in the tunnel for more than two hours, during which time significant disruption was caused to the East Coast Main Line. , the operators passing through the Welwyn Tunnels include Grand Central, Great Northern and Thameslink, Hull Trains, London North Eastern Railway, and Lumo.