The infrastructure and public transport systems in Switzerland are of very high quality. Owing to its location in the centre of Europe, Switzerland has a very dense network of roads and railways. The crossing of the Alps is a strategic issue for European transportation, as they separate the North from the South of the continent. The Swiss train service is very modern and reliable and connects all urban areas as well as providing links to abroad. There are also a widespread network of yellow post busses linking even the remotest rural areas with nearby towns and cities. Switzerland has more than 20 domestic airports and three international airports, with Zurich International Airport as the major hub linking the country to the rest of Europe and beyond.