The country's most important waterway is the Vistula river. The largest seaports are the Port of Gdańsk, the Port of Gdynia and the Port of Szczecin. Air travel is generally used for international travel, with many flights originating at Warsaw Chopin Airport. Railways connect all of Poland's major cities and the state-owned Polish State Railways (PKP) corporation, through its subsidiaries, runs a great number of domestic and international services of varying speed and comfort. In addition to this, five out of sixteen Polish voivodeships have their own provincial rail service providers.