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 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.
The X class locomotives are examples of 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.
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.
Locomotive No. 37385 is the basis for Ashima from Thomas and Friends.