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 reachees 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 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.

A rack railway is a steep grade railway with a toothed rack rail, usually between the running rails. The trains are fitted with one or more cog wheels or pinions that mesh with this rack rail. This allows the trains to operate on steep grades above 10%, which is the maximum for friction-based rail. Most rack railways are mountain railways, although a few are transit railways or tramways built to overcome a steep gradient in an urban environment.