The national road network has a total length of , of which are paved. Railroads are a major mode of freight transportation, although highways carry a progressively larger share of freight. Bulgaria has of railway track and currently a total of of high-speed lines are in operation. Rail links are available with Romania, Turkey, Greece, and Serbia, and express trains serve direct routes to Kyiv, Minsk, Moscow and Saint Petersburg. Sofia and Plovdiv are the country's air travel hubs, while Varna and Burgas are the principal maritime trade ports.