This is a good question with an interesting answer. The crushed stones are what is known as ballast. Their purpose is to hold the wooden cross ties in place, which in turn hold the rails in place.
Think about the engineering challenge faced by running miles of narrow ribbons of steel track on top of the ground: they are subject to heat expansion and contraction, ground movement and vibration, precipitation build-up from rough weather, and weed and plant growth from underneath.
Now keep in mind that while 99 per cent of the time they are just sitting there unburdened, the remaining 1 per cent they are subject to heavy moving loads.
Put all this together, and you have yourself a really, really interesting problem that was first solved nearly 200 years ago, and hasn't been significantly improved since!
The answer is to start with the bare ground, and then build up a foundation to raise the track high enough so it won't get flooded.
On top of the foundation, you deposit a load of crushed stone (the ballast). On top of the stone, you lay down (perpendicular to the direction of the track) a line of wooden beams. You then continue to dump crushed stone all around the beams. The sharp edges of the stone make it difficult for them to slide over each other (in the way that smooth, round pebbles would), thus effectively locking them in place. Next, you bring in hot-rolled steel rails... and lay them on top of the ties, end to end.
It would seem that you could just nail them or bolt them down to the ties, but that won't work. The non-trivial movement caused by heat expansion and contraction along the length of the rail would cause it to break or buckle if any of it were fixed in place. So, instead, the rails are attached to the sleepers by clips or anchors, which hold them down but allow them to move longitudinally as they expand or contract.
So there you have it: a centuries-old process that is extremely effective at facilitating the movement of people and material over thousands of miles... even though nothing is permanently attached to the ground with a fixed connection!
The ballast distributes the load of the ties (which in turn bear the load of the train on the track, held by clips) across the foundation, allows for ground movement, thermal expansion and weight variance, allows rain and snow to drain through the track, and inhibits the growth of weeds and vegetation that would quickly take over the track.
David S Rose, entrepreneur, investor, mentor
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