The X class locomotives are a class of metre gauge 0-8-2T rack and pinion compound locomotives working on the Nilgiri Mountain Railway in the Nilgiri Hills of southern India. They are used on the 20 km section between Coonoor and Mettupalayam where the gradient of the track can be as steep as 1 in 12.5. At these steep sections, the railway uses the Abt system. The X class locomotives are compound locomotives with two high-pressure and two low-pressure cylinders. All cylinders are outside the locomotive's frames. The two low-pressure cylinders drive the rack wheels. These cylinders are positioned above the two main high-pressure cylinders, which drive the main wheels.