|Nilgiri Mountain Railway X class|
Nilgiri Mountain Railway locomotive No.37385, preserved in the Delhi Railway Museum
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 wheels, and are positioned above the two main high-pressure cylinders which drive the main wheels.
This class of engine was acquired to replace the line's original Beyer-Peacock 2-4-0 engines, which were not powerful enough to handle the traffic. The X-class locomotives were bought in two batches from the Swiss Locomotive and Machine Works, Winterthur, Switzerland. The first batch of 12 was delivered between 1920 and 1925, and the second batch of five was delivered in 1952. Four more members of the class were built in India between 2011 and 2014.
|SLM Works Nos.||Year||Quantity||NMR No.||All India No.||Notes|
|2456–2459, 2469–2470||1914||6||1–6||37384||ex-No. 1|
|unknown||2011 - 2014||4||18-21||37396-37399||Non-SLM|
The coal-fired Locomotive No. 37395 was modified to oil firing in 2002, and another locomotive was similarly modified shortly afterwards. The railway intends to convert more locomotives to oil-fired system, since they are less likely to spark forest fires and are easier to refuel. Unlike coal-fired engines which require two firemen, only one fireman needs to travel with the driver of an oil-fired locomotive.
To ease the load on existing X-class locomotives, four oil-fired X-Class 0-8-2T rack and pinion compound engine steam locomotives with a similar design were ordered. The first one (No. X 37396) arrived in February 2011 and entered service on March 24 of that year. The second one (X 37397) was rolled out at the Golden Rock Railway Workshop in February 2012, and entered service on the railway in March. The third one built by Golden Rock, Trichy (X 37398), entered service in March 2013. The fourth (X 37399) was rolled out on March 5, 2014 at the Central Workshop in Ponmalai, Tiruchirapalli, to join the fleet after trials on the railway.
Ashima, a character based on the X Class and voiced by Tina Desai, appears in the 2016 animated film Thomas & Friends: The Great Race. She also appears in the twenty-second series and the twenty-third series of Thomas & Friends. 
Media related to Nilgiri Mountain Railway class X at Wikimedia Commons