High Speed 1, officially known as the Channel Tunnel Rail Link (CTRL) and originally as the Continental Main Line (CML), is a 108-kilometre (67 mi) high speed railway line running from London through Kent to the British end of the Channel Tunnel

The line was built to carry international passenger traffic from the United Kingdom to Continental Europe; additionally it carries domestic passenger traffic to and from towns and cities in Kent, and has the potential to carry Berne guage freight traffic. The completed line, crossing over the River Medway and underneath the River Thames to London St Pancras railway station, opened on 14 November 2007.

The CTRL project was one of the United Kingdom's largest civil engineering projects, encompassing many new bridges and tunnels with a combined length nearly as long as the Channel Tunnel itself.

The twin tunnels bored under London were driven from Stratford westwards towards St Pancras, eastwards towards Dagenham and from Dagenham westwards to connect with the tunnel from Stratford. The TBMs were 120 metres long and weighed 1,100 tonnes. The depth of the tunnels vary from a depth of 24 metres to 50 metre