X class locomotives are metre gauge 0-8-2T rack and pinion compound locomotives on 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 steepest gradient is 1 in 12.5. The railway uses the Abt system on these steep sections. The locomotives have two high-pressure and two low-pressure cylinders, located outside the locomotive's frames. The low-pressure cylinders drive the rack gears, and are positioned above the two main high-pressure cylinders which drive the main wheels.