The original plan for the Channel Tunnel Rail Link (CTRL) involved a tunnel from south-east of London to an underground terminus in the vicinity of King's Cross.
However, a late change of plan, principally driven by the then Secretary of State for the Environment Michael Heseltine's desire for urban regeneration in east London, led to a change of route, with the new line approaching London from the east.
This opened the possibility of reusing St Pancras as the terminus, with access via the North London Line, which crosses the throat of the station.

The idea of using the North London line was rejected in 1994 by the transport secretary, John MacGregor, as "difficult to construct and environmentally damaging".
However, the idea of using St Pancras station as the terminus was retained, albeit now linked by 12.4 miles (20 km) of new tunnels to Dagenham via Stratford.

London and Continental Railways (LCR), created at the time of British Rail privatisation, was selected by the government in 1996 to reconstruct St Pancras, build the CTRL, and take over the British share of the Eurostar operation.
 LCR had owned St Pancras station since privatisation to allow the station to be redeveloped.
Financial difficulties in 1998, and the collapse of Railtrack in 2001, caused some revision of this plan, but LCR retained ownership of the station.

The design and project management of reconstruction was undertaken on behalf of LCR by Rail Link Engineering (RLE), a consortium of Bechtel, Arup, Systra and Halcrow.
The original reference design for the station was by Nick Derbyshire, former head of British Rail's in-house architecture team.
The master plan of the complex was by Foster and Partners, and the lead architect of the reconstruction was Alistair Lansley, a former colleague of Nick Derbyshire recruited by RLE.

To accommodate 300-metre+ Eurostar trains, and to provide capacity for the existing trains to the Midlands and the new Kent services on the high-speed rail link, the train shed was extended a considerable distance northwards by a new flat-roofed shed.
The station was initially planned to have 13 platforms under this extended train shed.
East Midlands services would use the western platforms, Eurostar services the middle platforms, and Kent services the eastern platforms.
The Eurostar platforms and one of the Midland platforms would extend back into the Barlow train shed.
Access to Eurostar for departing passengers would be via a departure suite on the west of the station, and then to the platforms by a bridge above the tracks within the historic train shed.
Arriving Eurostar passengers would leave the station by a new concourse at its north end.

This original design was later modified, with access to the Eurostar platforms from below, using the station undercroft and allowing the deletion of the visually intrusive bridge.
By dropping the extension of any of the Midland platforms into the train shed, space was freed up to allow wells to be constructed in the station floor, which provided daylight and access to the undercroft.

The reconstruction of the station was recorded in the BBC Television documentary series The Eight Hundred Million Pound Railway Station broadcast as six 30-minute episodes between 13‒28 November 2007.
The original plan for the Channel Tunnel Rail Link (CTRL) involved a tunnel from south-east of London to an underground terminus in the vicinity of King's Cross. However, a late change of plan, principally driven by the then Secretary of State for the Environment Michael Heseltine's desire for urban regeneration in east London, led to a change of route, with the new line approaching London from the east. This opened the possibility of reusing St Pancras as the terminus, with access via the North London Line, which crosses the throat of the station.
The idea of using the North London line was rejected in 1994 by the transport secretary, John MacGregor, as "difficult to construct and environmentally damaging". However, the idea of using St Pancras station as the terminus was retained, albeit now linked by 12.4 miles (20 km) of new tunnels to Dagenham via Stratford.
London and Continental Railways (LCR), created at the time of British Rail privatisation, was selected by the government in 1996 to reconstruct St Pancras, build the CTRL, and take over the British share of the Eurostar operation.  LCR had owned St Pancras station since privatisation to allow the station to be redeveloped. Financial difficulties in 1998, and the collapse of Railtrack in 2001, caused some revision of this plan, but LCR retained ownership of the station.
The design and project management of reconstruction was undertaken on behalf of LCR by Rail Link Engineering (RLE), a consortium of Bechtel, Arup, Systra and Halcrow. The original reference design for the station was by Nick Derbyshire, former head of British Rail's in-house architecture team. The master plan of the complex was by Foster and Partners, and the lead architect of the reconstruction was Alistair Lansley, a former colleague of Nick Derbyshire recruited by RLE.
To accommodate 300-metre+ Eurostar trains, and to provide capacity for the existing trains to the Midlands and the new Kent services on the high-speed rail link, the train shed was extended a considerable distance northwards by a new flat-roofed shed. The station was initially planned to have 13 platforms under this extended train shed. East Midlands services would use the western platforms, Eurostar services the middle platforms, and Kent services the eastern platforms. The Eurostar platforms and one of the Midland platforms would extend back into the Barlow train shed. Access to Eurostar for departing passengers would be via a departure suite on the west of the station, and then to the platforms by a bridge above the tracks within the historic train shed. Arriving Eurostar passengers would leave the station by a new concourse at its north end.
This original design was later modified, with access to the Eurostar platforms from below, using the station undercroft and allowing the deletion of the visually intrusive bridge. By dropping the extension of any of the Midland platforms into the train shed, space was freed up to allow wells to be constructed in the station floor, which provided daylight and access to the undercroft.
The reconstruction of the station was recorded in the BBC Television documentary series The Eight Hundred Million Pound Railway Station broadcast as six 30-minute episodes between 13‒28 November 2007.
St Pancras railway station