Nilgiri Mountain Railway X class. The X class is a metre gauge   rack and pinion compound locomotive used by the Nilgiri Mountain Railway in the Nilgiri Hills of southern India. They are used on the  section between Coonoor and Mettupalayam, where the line reaches 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 of their frames. The low-pressure cylinders drive the rack gears, 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 & Company  rack locomotives, which were not powerful enough to handle the traffic. They were bought in two batches from the Swiss Locomotive & Machine Works, Winterthur, Switzerland. The first batch of twelve was delivered between 1914 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, and another two between 2021 and 2022.

Class table

Conversion to oil 

The coal-fired No. 37395 was modified to fire oil in 2002, and another locomotive was similarly modified shortly afterwards. The railway intends to convert more locomotives to a 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. Since Aprill 2022, all oil-fired steam engines run on High-Speed Diesel (HSD), as it is less viscous and contains less sulfur, compared to furnace oil.

New locomotives 

To ease the load on existing X-class locomotives, four new oil-fired X Class steam locomotives to the same basic design were ordered. The first one (No. X 37396), named Neela Kurinji, arrived in February 2011 and entered service on March 24 of that year. The second (No. X 37397), Betta Queen, was rolled out at the Golden Rock Railway Workshop in February 2012, and entered service on the railway in March. The third (No. X 37398), also from the Golden Rock Workshop, Nilgiri Queen, entered service in March 2013. The fourth (No. X 37399), named Nilgiri Flycatcher was rolled out on March 5, 2014, to join the fleet after trials on the railway.

Under the order of the Central Government, the Golden Rock Workshop started the manufacturing of two X-Class steam engines, one coal-powered and one oil-powered, in 2020. However, due to COVID-19, they could not be finished on time. (Source:- Deccan Herald). X-37400, the coal-powered engine, was finally rolled out on August 25, 2021. The oil-fired locomotive, X-37401, rolled out on September 26, 2022. Both are currently in service.

In fiction
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, the twenty-third series and the twenty-fourth series of Thomas & Friends.

In Bollywood

The X-class steam engines were temporarily repainted and used in the music video for the song Chaiyya Chaiyya, sung by Sukhwinder Singh and Sapna Awasthi, with music composed by A. R. Rahman and lyrics by Gulzar. The video features Shah Rukh Khan and Malaika Arora dancing on the train, from the 1998 Mani Ratnam movie Dil Se. For the song, Ratnam spoke to NMR in regards to painting the engines greenish-black and the train rakes a brick color for five days, while the music video was shot.

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Steam locomotives of India
Compound locomotives
0-8-2T locomotives
SLM locomotives
Rack and cog driven locomotives
Metre gauge steam locomotives
Passenger locomotives
Railway locomotives introduced in 1914