was allowed to enter the tunnel at a time. In June 1852, a fatal collision occurred between a ballast train and a coal train as a result of signals that were either not issued or were not received. In 1879, the single track was doubled. In March 1987, both the north and south portals of Kilsby Tunnel were Grade II* listed; furthermore, the two largest ventilation shafts were also listed, the north shaft being Grade II* listed while the south shaft was Grade II listed. During the tunnel's service life, there have been no major problems or difficulties with the structure. During a survey conducted during the 2010s, it was found to be in a largely good condition. During the 2010s, Kilsby Tunnel was restored by Network Rail. Maintenance focused on inspecting the ventilation shafts and repairs to the brickwork of one shaft was carried out, sections of the tunnel's lining were replaced. While unusual means of access were used because of the listed status of the shafts, the repairs were described as routine. At the beginning of 2020, an estimated 400 trains were using the tunnel daily. Due to the drastic reduction in rail traffic caused by the COVID-19 pandemic, Network Rail took the rare opportunity to close the tunnel for two weeks in May 2020 in order to carry out major works, including replacing much of the tunnel's drainage system, which had become clogged by heavily limed water seeping through, causing flooding problems, and also replacing much of the track and ballast. This was the longest total closure of the tunnel since it was opened, with all trains diverted temporarily via . In normal times the work would have required many short blockades, and taken months to complete. It allowed a temporary speed restriction through the tunnel of 70 mph to be lifted, enabling trains to operate at the normal line speed of 110 mph. Notes References External links Kilsby Tunnel includes the classic image of the tunnel under construction. A short video of an air shaft by Si Horton using data collected by Colin Humphreys using a Faro Focus X330 laser scanner (on YouTube) Rail transport in Northamptonshire Buildings and structures in Northamptonshire Railway tunnels in England Tunnels in Northamptonshire Tunnels completed in 1838 London and Birmingham Railway