Why do railway tracks have crushed stones alongside them?
This article doesn't explain one interesting bit:<p>It is important that these crushed stones stay <i>clean</i>.    If dirt gets in them, known as fouling, filling the gaps between the stones, then eventually it will cause a derailment.<p>Briefly, the stones move slightly as each train passes above them.  The tracks 'float' on the denser stone.  This effect helps keep the track straight and level - higher regions get pushed down, while lower regions raise themselves up.<p>Fouling stops this effect, and eventually the track will become so uneven that a derailment happens.<p>To prevent this, a big part of railway maintenance involves digging out all the stones, washing them to get rid of sand and soil, and putting them back.   Usually done every 25 years or so.