Why do Indian trains make a “tchjk tchjk tchjk tchjk” sound?

Due to a lot of reasons:

Rails-wheel metal on metal collision sound: The rail track laid out is not a made of 2 single piece of iron rod each, but a lot of pieces together. This is because metal expands and contracts when there’s temperature fluctuations and if it’s a single piece, during summer when the temperature increases, the rails will expand and bend. Now even though these tracks may seem even in height, there are slight irregularities that are there in the level or height of these individual pieces. When an approaching train wheel hits this uneven rail and passes over it, it makes metal hitting on metal sound, ‘tjick’. Since there are 2 pairs of 4 wheel each in a coach, the sound is ‘tchjk tchjk.’

Spring compression and expansion: The 2 pairs of 4 wheels explained earlier had 2 giant springs attached to each other. These springs compress when the train wheel passes over an uneven rail piece that is slightly higher than the previous rail and the spring expands when the uneven rail piece is lower in height than the previous one. The frequency of such expansion is very high. This also contributes to the ‘tchjk tchjk’ sound.

Vibration of other rail components: Rails are held together with the ballast and concrete sleepers with metallic clamps. These things vibrate vigorously when a train is approaching or passing over it. This is why you can hear ‘tchjk tchjk’ sound even when the train is far from you and approaching you.

Squeaking sound when metal wheel slides on metal rails: Even though the wheels are lubricated for smooth sliding over the rails, it produces a squeaking sound when sliding over each other. This happens especially when train takes curves.

Echo amplification by the coaches: Since the bottom part of the coaches are shaped like empty boxes, the sound produced by the above 4 pointers are trapped inside and amplified by the coaches itself due to echoing.

And there you go.. Put everything together and you end up with the famous sound that A R Rahman stole for many of his songs to win Oscars!!!