The X class locomotives are a class of metre gauge0-8-2T 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.
Engines of this class were first brought as replacements for the line's original Beyer-Peacock 2-4-0 engines whose traction power was too weak for the traffic. The X class locos were bought in two batches from the Swiss Locomotive and Machine Works, Winterthur, Switzerland. The first batch (12-nos.) were brought into operation over the years between 1920 and 1925 and the newer batch (5-nos.) in 1952.
Four new oil-fired steam locomotives have been ordered to ease the load on the existing X class locomotives. They are basically the same design as the older locomotives. The first of these, No. X 37396, arrived in Feb 2011 and entered service on March 24, 2011. The second oil fired steam engine, No. X 37397, was rolled out at Golden Rock in Feb. 2012, and is to enter service on NMR in March 2012. The third indigenous steam engine built by Golden Rock Workshop, Trichy, X 37398 entered service in March 2013. The last of four oil fired steam engines of X Class 0-8-2T rack and pinion compound engine bearing road number X 37399 was rolled out on March 5, 2014 at the Central Workshop, Ponmalai, Tiruchirapalli. It is expected to join the fleet after trials on the rack railway.
Nilgiri Mountain Railway locomotive No.37395 at Coonor Shed, February 2005
One locomotive, No. 37395, was modified from being coal to oil-fired in 2002, another has been modified and the Railway is looking to retro-fit further locomotives to oil-firing. Conversion to an oil-fired system has the potential to reduce the danger of forest fires that could be caused by ashes flying out from the locomotive. Additionally, the effort involved in the manual lifting of coal and putting it into the boiler is eliminated. The current oil-fired locomotives need only one fireman to accompany the driver, while coal-fired engines need two firemen. However, X 37395 was reconverted to coal-burner in August 2009 for no reasons.