Serbia has 3,819 kilometres of rail tracks, of which 1,279 are electrified and 283 kilometres are double-track railroad. The major rail hub is Belgrade (and to a lesser degree Niš), while the most important railroads include: Belgrade–Subotica–Budapest (Hungary) (currently upgraded to high-speed status), Belgrade–Bar (Montenegro), Belgrade–Šid–Zagreb (Croatia)/Belgrade–Niš–Sofia (Bulgaria) (part of Pan-European Corridor X), and Niš–Thessaloniki (Greece). Although still a major mode of freight transportation, railroads face increasing problems with the maintenance of the infrastructure and lowering speeds. Rail services are operated by Srbija Voz (passenger transport) and Srbija Kargo (freight transport).