London has one of the oldest and largest public transport systems in the world. Many components of its transport system, such as the double-decker bus, the Hackney Carriage black taxi and the Tube, are internationally recognised symbols of London. London has a comprehensive rail network with several major railway stations, including two international train stations at St Pancras railway station and Stratford International, which connect London to mainland Europe through the Eurostar service. London also has six international airports. In addition to public transport, London is the start point for a number of motorway routes. The M25 is an orbital motorway which enables vehicles to avoid travelling through central London, and is one of the busiest motorways in Europe.