UNESCO World Heritage rack railway - seats sell out on weekends, holidays and the Mar-Jun peak, so book on IRCTC as early as your dates allow. Services can be delayed or suspended after heavy rain. The Ooty-Coonoor short leg is a good fallback if the full route is sold out.
This 46-km metre-gauge line from Mettupalayam to Ooty was proposed in 1854 but only built between 1891 and 1908 due to the steep terrain, using Swiss Alpine rack-and-pinion technology (Abt system) on its steepest stretch between Mettupalayam and Coonoor. It is the only rack railway in India, originally hauled by Swiss-built X Class steam locomotives now manufactured at the Golden Rock Workshop in Tiruchirappalli. In July 2005, UNESCO extended the World Heritage designation of the Darjeeling Himalayan Railway to include it, creating the joint listing 'Mountain Railways of India'.