The Nilgiri Mountain Railway operates between Mettupalayam and Udagamandalam, better known as Ooty, in Tamil Nadu.
One of its most distinctive engineering features is the rack-and-pinion system used on steep sections between Mettupalayam and Coonoor.
A toothed rack positioned between the running rails works with specialised locomotive equipment to provide additional traction.
This allows trains to safely climb gradients that would be extremely difficult using conventional wheel-to-rail adhesion alone.
The railway forms part of UNESCO's Mountain Railways of India World Heritage property.