Wikipedia
T-Centralen (Swedish for "The T-Central"; T being an abbreviation for "tunnelbana", the Swedish word for "underground" or "subway") is a metro station that forms the heart of the Stockholm metro system, in the sense that it is the only station where all three of the system's lines meet. That, its central location, and its connections with other modes of transport make it the most used metro station in Stockholm. The station is located in the Norrmalm borough of Stockholm, between Sergels torg (Sergel's Square) and the street of Vasagatan.
On a winter day in 2012, some 330,000 passengers travelled to or from this station. It is connected by a pedestrian underpass to the neighbouring Stockholm Central Station across Vasagatan (for national and regional trains, and the Stockholm commuter rail network) and to the Cityterminalen long-distance bus terminal, making it easy to continue a journey started by metro train.
When opened on 24 November 1957 the name of the station was "Centralen" ("The Central"), but it was renamed on 27 January 1958, as the metro station often was mistaken for the central railway station to which it is connected. T-Centralen has two separate sets of platforms, connected by a long moving walkway on a mezzanine level.
A new station level is being built under the existing T-Centralen station, as a part of the Stockholm City Line. It will be used for local railway trains, and will have direct escalators to the metro platforms, and it is scheduled to open in 2017. It will probably not be a formal part of the metro station and probably will be called Stockholm City.