The passenger station was designed by William Henry Barlow and constructed on a site that had previously been a slum called Agar Town.

The approaching line to the station crossed the Regent's Canal at height allowing the line reasonable gradients; this resulted in the level of the line at St Pancras being 20 ft (6.1 m) above the ground level.
Initial plans were for a two or three span roof with the void between station and ground level filled with spoil from tunnelling to join the Midland Main Line to the St. Pancras branch.
Instead, due to the value of the land in such a location the lower area was used for freight, in particular beer from Burton.
As a result, the undercroft was built with columns and girders, maximising space, set out to the same plans as those used for beer warehouses, and with a basic unit of length that of a beer barrel.

The contract for the construction of the station substructure and connecting lines was given to Messrs. Waring, with Barlow's assistant Campion as supervisor.
The lower floor for beer warehousing contained interior columns 15 ft (4.57 m) wide, and 48 ft (14.63 m) deep carrying girders supporting the main station and track.
The connection to the Widened Lines (St. Pancras branch) ran below the station's bottom level, in an east-to-west direction.

To avoid the foundations of the roof interfering with the space beneath, and to simplify the design, and minimise cost, it was decided to construct a single span roof, with cross ties for the arch at the station level.
The arch was sprung directly from the station level, with no piers.
Additional advice on the design of the roof was given to Barlow by Rowland Mason Ordish.
The arches' ribs had a web depth of 6 ft (1.8 m), mostly open ironwork.
The span width, from wall to wall was 245 ft 6 in (74.83 m), with a rib every 29 ft 4 in (8.94 m) The arch was a slightly pointed design, with a reduced radius of curvature at the springing points.
The Butterley Company was contracted to construct the arches.
The total cost of the 24 rib roof and glazing was over £53,000, of which over half was for the main ribs.
The cost of the gable end was a further £8,500.

The single-span overall roof was the largest such structure in the world at the time of its completion.
The materials used were wrought iron framework of lattice design, with glass covering the middle half and timber (inside)/slate (outside) covering the outer quarters.
The two end screens were glazed in a vertical rectangular grid pattern with decorative timber cladding around the edge and wrought iron finials around the outer edge.
It was 689 feet (210.01 m) long, 240 feet (73.15 m) wide, and 100 feet (30.48 m) high at the apex above the tracks.

Local services began running to the Metropolitan Railway junction underneath the terminus on 13 July 1868.
The station itself opened to the public on 1 October.
The first service was an overnight mail train from Leeds.
The passenger station was designed by William Henry Barlow and constructed on a site that had previously been a slum called Agar Town.
The approaching line to the station crossed the Regent's Canal at height allowing the line reasonable gradients; this resulted in the level of the line at St Pancras being 20 ft (6.1 m) above the ground level. Initial plans were for a two or three span roof with the void between station and ground level filled with spoil from tunnelling to join the Midland Main Line to the St. Pancras branch. Instead, due to the value of the land in such a location the lower area was used for freight, in particular beer from Burton. As a result, the undercroft was built with columns and girders, maximising space, set out to the same plans as those used for beer warehouses, and with a basic unit of length that of a beer barrel.
The contract for the construction of the station substructure and connecting lines was given to Messrs. Waring, with Barlow's assistant Campion as supervisor. The lower floor for beer warehousing contained interior columns 15 ft (4.57 m) wide, and 48 ft (14.63 m) deep carrying girders supporting the main station and track. The connection to the Widened Lines (St. Pancras branch) ran below the station's bottom level, in an east-to-west direction.
To avoid the foundations of the roof interfering with the space beneath, and to simplify the design, and minimise cost, it was decided to construct a single span roof, with cross ties for the arch at the station level. The arch was sprung directly from the station level, with no piers. Additional advice on the design of the roof was given to Barlow by Rowland Mason Ordish. The arches' ribs had a web depth of 6 ft (1.8 m), mostly open ironwork. The span width, from wall to wall was 245 ft 6 in (74.83 m), with a rib every 29 ft 4 in (8.94 m) The arch was a slightly pointed design, with a reduced radius of curvature at the springing points. The Butterley Company was contracted to construct the arches. The total cost of the 24 rib roof and glazing was over £53,000, of which over half was for the main ribs. The cost of the gable end was a further £8,500.
The single-span overall roof was the largest such structure in the world at the time of its completion. The materials used were wrought iron framework of lattice design, with glass covering the middle half and timber (inside)/slate (outside) covering the outer quarters. The two end screens were glazed in a vertical rectangular grid pattern with decorative timber cladding around the edge and wrought iron finials around the outer edge. It was 689 feet (210.01 m) long, 240 feet (73.15 m) wide, and 100 feet (30.48 m) high at the apex above the tracks.
Local services began running to the Metropolitan Railway junction underneath the terminus on 13 July 1868. The station itself opened to the public on 1 October. The first service was an overnight mail train from Leeds.
St Pancras railway station