The X class is a metre gauge 0-8-2RT rack and pinion compound locomotive used by the Nilgiri Mountain Railway in the Nilgiri Hills of southern India. They are used on the 28-kilometre-long (17 mi) section between Coonoor and Mettupalayam, where the line reaches a gradient of 8%. The railway uses the Abt system on these steep sections. The locomotives have two high-pressure and two low-pressure cylinders, located outside of their frames. The low-pressure cylinders drive the rack gears, and are positioned above the two main high-pressure cylinders, which drive the main wheels.