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The train is small and open . It carries you into a hill near the town of Postojna , in the region of Inner Carniola in Slovenia . Behind you , the daylight disappears . You are inside Postojna Cave . According to the cave 's official website , the whole cave system is 24 kilometres long , and the train journey covers 3.7 kilometres of it . The River Pivka carved these tunnels over millions of years . Inside , the air stays at about 10 degrees all year , so a warm jacket is a good idea even in August . The railway has a long history . The official website says it opened on 16 June 1872 , as the first railway in an underground cave . At first there were no engines : guides pulled two small carriages , and each one carried four visitors . In 1923 , the line was extended and a petrol locomotive took over . In 1956 , the cave bought two battery - powered electric locomotives . When the train stops , the walking part of the tour begins . White columns rise from the floor to the roof . The most famous piece is a bright white stalagmite called the Brilliant , which is five metres tall . Water drips down it and leaves a thin layer of new stone , which keeps it shining . Near the end , you reach a tank of water and a pale , thin animal . This is the olm , a salamander that lives its whole life in water . Its eyes are covered by skin , so it can not see . According to the website , olms can live to 100 and go for years without food . In 1689 , the writer Johann Weikhard von Valvasor wrote about the olm . At that time , local people believed that olms , washed out of caves by heavy rain , were baby dragons . Then you climb back into the train , and it carries you out of the hill , into the daylight again . 
