Do you know which track is known as the Greatest Narrow-Gauge Engineering Marvel in India It is the 99-kilometer-long Shimla-Kalka railway line. Built in 1903 under the British, the train chugs along acres of lush greenery, over multiple bridges and through 100 tunnels over this track. In 2008, UNESCO added the Kalka&ndashShimla railway to the mountain railways of India World Heritage Site celebrating the cultural legacy of the country.