Brussels grew from a small rural settlement on the river Senne to become an important city-region in Europe. Since the end of the Second World War, it has been a major centre for international politics and home to numerous international organisations, politicians, diplomats and civil servants. Brussels is the "de facto" capital of the European Union, as it hosts a number of principal EU institutions, including its administrative-legislative, executive-political, and legislative branches (though the judicial branch is located in Luxembourg, and the European Parliament meets for a minority of the year in Strasbourg). Its name is sometimes used metonymically to describe the EU and its institutions. The secretariat of the Benelux and headquarters of NATO are also located in Brussels. As the economic capital of Belgium and one of the top financial centres of Western Europe with Euronext Brussels, it is classified as an "Alpha" global city. Brussels is a hub for rail, road and air traffic, sometimes earning the moniker "Crossroads of Europe". The Brussels metro is the only rapid transit system in Belgium. In addition, both its airport and railway stations are the largest and busiest in the country.