Gravel and Ballast on Rail Track

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BTW: your assumption is fundamentally incorrect.

Thanks

Gravels, of different sizes, and complete underlaid pebbles is the most economical way of preventing sub-stratum soil erosion which in turn prevents rail tracks from sinking. Gravels let all water thru it and hence does not derail tracks. But having said, not all tracks can have this. For example, in Rajasthan desert, the tracks are well laid on concrete base, which is generally time consuming and expensive, at least so far.

And gravels have another advantage that if certain area is washed out, it can be easily replenished without problems.

http://en.wikipedia.org/wiki/Track_ballast