The National Rail network of 10,072 route miles (16,116 km) in Great Britain and 189 route miles (303 route km) carries over 18,000 passenger and 1,000 freight trains daily. Urban rail networks exist Birmingham, Liverpool, London, Manchester and Newcastle. There are many regional and international airports, with Heathrow Airport in London being one of the busiest in the world. The UK also has a network of ports which received over 558 million tons of goods in 2003–2004. Transport is the largest source of greenhouse gas emissions by the United Kingdom.