the spectacular Flåm line, Norway

Although only 20 km long, the railway line between Myrdal on the Bergen railway (at 865 metres altitude) and the near sea level station at Flåm, in one of Norway's spectacular fjords, probably has more ridership than any other non-urban line in the country, as tour buses and cruise ships give their passengers a ride on it as part of their trips.  It follows a tortuous route with 20 tunnels involving horseshoe curves and one helicoid. The line celebrated its 70th anniversary last year.

It was part of the Norwegian State Railway or NSB system until 1998 when it was privatised.

Above is a 1950s postcard view, and below is the map from the company's website.