On 21 June 1838, resident engineer Charles Lean laid the final brick of the tunnel, marking its completion.
It took three years, and cost £320,000 to build — three times the original estimate (£23,758,000 in 2005 pounds).
The high accident rate in the course of its construction included the deaths of 26 of the 1,250 workers.
The length of time it took to build delayed the opening of the L&BR.
On 20 August 1838, the directors of the L&BR conducted the first ever rail journey between Birmingham Curzon Street railway station and Euston railway station, London.
On the way, the train briefly stopped at the tunnel so that they could marvel at the structure and meet some of the workmen.
Regular services using the tunnel commenced soon after.

Kilsby Tunnel had been designed for double tracks, but opened with a single track.
During early operations, train movements were controlled by two policeman, one at either end of the tunnel, who signalled the presence of a train to each other, and only one train 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 Northampton.
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.
On 21 June 1838, resident engineer Charles Lean laid the final brick of the tunnel, marking its completion. It took three years, and cost £320,000 to build — three times the original estimate (£23,758,000 in 2005 pounds). The high accident rate in the course of its construction included the deaths of 26 of the 1,250 workers. The length of time it took to build delayed the opening of the L&BR. On 20 August 1838, the directors of the L&BR conducted the first ever rail journey between Birmingham Curzon Street railway station and Euston railway station, London. On the way, the train briefly stopped at the tunnel so that they could marvel at the structure and meet some of the workmen. Regular services using the tunnel commenced soon after.
Kilsby Tunnel had been designed for double tracks, but opened with a single track. During early operations, train movements were controlled by two policeman, one at either end of the tunnel, who signalled the presence of a train to each other, and only one train 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 Northampton. 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.
Kilsby Tunnel