The Nilgiri Mountain Railway is one of India's most impressive mountain railway engineering achievements.
UNESCO records that the line climbs from approximately 326 metres above sea level to around 2,203 metres.
That represents an elevation increase of nearly 1,900 metres.
To overcome particularly steep gradients, portions of the railway use an unusual rack-and-pinion traction arrangement.
The approximately 46-kilometre metre-gauge route was completed to Udagamandalam, popularly known as Ooty, in 1908.
Its extreme elevation change, historic engineering and continuing operation helped earn it UNESCO World Heritage recognition.