Within a year of the new station's opening, a pair of tunnels was added, which surfaced on the GNR just north of King's Cross and provided a direct rail connection between the two lines.

In 1868 a second pair of tracks, known as the City Widened Lines, was opened along with a tunnel connection to the Midland Railway near St Pancras station.

In the 1980s the City Widened Lines were electrified and the Snow Hill tunnel reopened to passenger traffic as part of the Thameslink programme.

[2] The Metropolitan line part of the station was renamed to King's Cross & St Pancras in 1925 and then to King's Cross St Pancras in 1933,[3] when the Metropolitan Railway was merged with the Underground Electric Railways Company of London to form the London Passenger Transport Board (LPTB).

King's Cross Midland City, as it was then called, was one of the original eighteen stations in the group,[11] and it retained this status until closure in 2007.

[15] Plans for the station were made in December 1848 under the direction of George Turnbull, resident engineer for constructing the first 20 miles (32 km) of the Great Northern Railway out of London.

[14] The first suburban services to and from King's Cross began operating in 1861, initially to Seven Sisters Road station (which was renamed to Finsbury Park in 1869), and later to Hornsey and beyond.

The Metropolitan was able to continue operating by leasing rolling stock from the GNR, which it brought on to the line through the York Road tunnel.

These ran alongside the original tracks from King's Cross through to Moorgate,[25] and allowed GNR and Metropolitan traffic to run along the line simultaneously.

[26] The same year the Metropolitan built a pair of tunnels to the Midland Railway tracks north of its new terminus at St Pancras station.

[30] The route through King's Cross Metropolitan remained busy throughout the remainder of the nineteenth century, with trains from five companies—the Metropolitan, GNR, Midland, LCDR and the South Eastern Railway (SER)—and routes including Victoria to Peterborough as well as services from north London to Chatham and Dover.

This became known as the King's Cross Suburban station, but suffered from several problems including a steep incline and sharp curve, along with a build-up of smoke because of its proximity to the mouth of the tunnel.

[35] The advent of deep-level tube lines at the turn of the twentieth century caused major changes in the underground network.

[36] The tube platforms were linked to King's Cross Metropolitan through the same foot tunnel as the mainline, making one station complex.

[24] The Snow Hill tunnel closed to passenger service during the war, and the north–south link was used only by freight in the postwar years.

The Circle line between Euston Square and King's Cross was particularly damaged and services stopped completely for five months.

[42] In 1979 an £80 million project was launched to electrify the Midland Mainline from Bedford to St Pancras and the City Widened Lines.

Trains ran between Bedford and Moorgate, via St Pancras and the tunnels from the Midland mainline to the City Widened Lines.

[44] In 1988 Network SouthEast, one of the newly created sectors of the state-owned British Rail,[45] implemented a scheme first proposed in the 1960s to reopen the Snow Hill tunnel to passenger traffic.

[47] This plan was developed in some detail by architect Norman Foster, but was ultimately vetoed by the government in 1990 due to high costs.

[48] The project, which became known as High Speed 1, was eventually completed in 2007 with terminal platforms at St Pancras rather than King's Cross.

They criticised its substandard platform widths and lengths, lack of step-free access and fire escape routes, as well as the poor-quality passenger environment.

The surrounding infrastructure made this impractical and it would also have caused serious disruption to the Circle, Hammersmith & City and Metropolitan Underground lines as well as nearby roads.

Ultimately though, due to the cost of relocating the lines and the political issues with demolishing the Grade-II-listed Great Northern Hotel, Network Rail decided to build the new platforms on the existing alignment, under the St Pancras complex.

In November 2021 Transport for London advised in response to a Freedom of Information request that it had no immediate plans to reopen it, because they were in the process of reviewing the utility of the entrance in light of the fact that the ticket office building requires structural improvement works.