Suspension generally used to make ride comfortable. But rail tracks are smooth and even; intution says trains no need suspension. But they have it.
Why do railway wagon/coach require suspension when the tracks are so smooth and flat?
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Tracks are not smooth and flat. There will always be slight misalignments between neighbouring track spans. Suspension is not only to make ride comfortable, it's also to shield the frame of the car from huge impulses that would arise from even small bumps: otherwise the repeated impulses would wear the structure out and lead to stress cracking very swiftly.
The "clickety-clack" sound of the train you hear is these small impulsive bumps as the wheels roll over the joints. These are so impulsive and of such small amplitude that almost any large structure like a railcar would absorb them so that you could barely feel them in your seat, even without suspension. But they are very damaging to the frame, so their impulse is reduced by allowing the wheels to move independently of the frame on springs.