fragmentation, along with increased movement of people since the industrial revolution, has led to a high level of cooperation between European countries in developing and maintaining transport networks. Supranational and intergovernmental organisations such as the European Union (EU), Council of Europe and the Organization for Security and Co-operation in Europe have led to the development of international standards and agreements that allow people and freight to cross the borders of Europe, largely with unique levels of freedom and ease. Rail transport Rail networks in Western and Central Europe are often well maintained and well developed, whilst Eastern, Northern and Southern Europe often have less coverage and/or infrastructure problems. Electrified railway networks operate at a plethora of different voltages AC and DC varying from 750 to 25,000 volts, and signalling systems vary from country to country, hindering cross-border traffic. EU rail subsidies amounted to €73 billion in 2005. Air transport Despite an extensive road and rail network, most long-distance travel within Europe is by air. A large tourism industry also attracts many visitors to Europe, most of whom arrive into one of Europe's many large international airports. London is the second busiest airport in the world by number of international passengers, only trailing Dubai. The advent of low cost carriers in recent years has led to a large increase in air travel within Europe. Air transportation is now often the cheapest way of travelling between cities. This increase in air travel has led to problems of airspace overcrowding and environmental concerns. The Single European Sky is one initiative aimed at solving these problems. Global trade relations The bulk of the EU's external trade is done with China, Mercosur and the United States, Japan, Russia and non-member European states. EU members are represented by a single official at the WTO. The EU is involved in a few minor trade disputes. It had a long running dispute with the USA of allegedly unfair subsidies the US government gives to several companies, such as Boeing. The EU has a long running ban prohibiting arms trade with the Chinese. The EU issued a brief accusing Microsoft of predatory and monopolistic practices. See also Free trade areas in Europe Golden Banana Cryptocurrencies in Europe Culture of Europe List of companies of the European Union List of largest European companies by revenue Regions of Europe Statistics International organisations in Europe List of European countries by budget revenues