The North–South connection (, ) is a railway link of national and international importance through central Brussels, Belgium, that connects the major railway stations in the city. It is line 0 (zero) of the Belgian rail network. With 1200 trains a day, it is the busiest railway line in Belgium and the busiest railway tunnel in the world. It has six tracks and is used for passenger trains, or rarely for a maintenance train when work is to be done on the railway infrastructure inside the North–South connection itself, but not for freight trains. It is partially underground (around Brussels Central Station) and partially raised above street level. History During the late 19th and early 20th centuries, Brussels was served by two main railway stations: Brussels-North (opened in 1846) and Brussels-South (opened in 1869, replacing a nearby station of 1840). They are located just outside opposite ends of the Pentagon—an area within the ring roads which follow the boundary of the old city walls. Shortly after opening, both stations were handling large volumes of commuter, regional and international passengers, but through journeys required disembarking and a street-level transfer through the city's old town, a distance of over . The idea of an underground railway line linking the two stations was first suggested in the 1860s, as part of a proposal for the covering of the Senne, although it was never implemented. The current version was planned before World War II, after a decision originally made in 1909, and it came into service on 5 October 1952. Both stations were demolished and reconstructed to allow through services, reopening in 1952. Three new intermediate stations were constructed along the route to serve the city centre. Two of them, Brussels-Chapel and Brussels-Congress, were intended stops only for local commuter services and have never been heavily used. The largest of the new stations, Brussels-Central, was built to additionally serve regional and international services transiting through Brussels. The combination of a city-centre location and numerous services to diverse destinations led to Brussels-Central becoming the busiest station in Belgium. Brussels-North, Brussels-Central and Brussels-South are now the three main railways stations in the city; they are also the three busiest stations in all of Belgium. Between 2018 and 2019, the North–South connection's tunnel was renovated to improve ventilation and smoke extraction in the event of a fire. The six-lane underground tunnel, separated by the pillars supporting the