The Norwegian railway network consists of 2,552 kilometers (1,586 mi) of electrified railway lines, constituting 62% of the Norwegian National Rail Administration's 4,114 kilometers (2,556 mi) of line. The first three mainline systems to be electrified were private ore-hauling lines. The Thamshavn Line opened in 1909, and remained in revenue use until 1973, after which it was converted to a heritage railway. It is the world's oldest remaining alternating-current railway and the only narrow gauge railway in the country to have been electrified. It was followed by Norsk Transport's Rjukan and Tinnoset Lines two years later, and Sydvaranger's Kirkenes–Bjørnevatn Line in 1922. The Norwegian State Railways' (NSB) first electrification was parts of the Drammen Line in 1922 and the ore-hauling Of